[ENH] Extend Extension class test suite (#1560)

#### Metadata
* Reference Issue: fixes #1545 
* New Tests Added: Yes
* Documentation Updated: No
* Change Log Entry: Add tests for extension interface contract and extension registry edge cases

#### Details 
* What does this PR implement/fix? Explain your changes.
This PR adds unit tests for the OpenML Extension interface and for extension registry behavior. The tests added are the 7 tests mentioned in #1545 
* Why is this change necessary? What is the problem it solves?
Previously, only the non-abstract registry helpers (`get_extension_by_model`, `get_extension_by_flow`) were covered. The abstract `Extension` interface itself was not tested.

Author: satvshr

tests/test_extensions/test_functions.py

@@ -1,12 +1,14 @@
 # License: BSD 3-Clause
 from __future__ import annotations
 
-import inspect
+from collections import OrderedDict
 
+import inspect
+import numpy as np
 import pytest
-
+from unittest.mock import patch
 import openml.testing
-from openml.extensions import get_extension_by_flow, get_extension_by_model, register_extension
+from openml.extensions import Extension, get_extension_by_flow, get_extension_by_model, register_extension
 
 
 class DummyFlow:
@@ -40,54 +42,197 @@ def can_handle_model(model):
         return False
 
 
-def _unregister():
-    # "Un-register" the test extensions
-    while True:
-        rem_dum_ext1 = False
-        rem_dum_ext2 = False
-        try:
-            openml.extensions.extensions.remove(DummyExtension1)
-            rem_dum_ext1 = True
-        except ValueError:
-            pass
-        try:
-            openml.extensions.extensions.remove(DummyExtension2)
-            rem_dum_ext2 = True
-        except ValueError:
-            pass
-        if not rem_dum_ext1 and not rem_dum_ext2:
-            break
+class DummyExtension(Extension):
+    @classmethod
+    def can_handle_flow(cls, flow):
+        return isinstance(flow, DummyFlow)
+
+    @classmethod
+    def can_handle_model(cls, model):
+        return isinstance(model, DummyModel)
+
+    def flow_to_model(
+        self,
+        flow,
+        initialize_with_defaults=False,
+        strict_version=True,
+    ):
+        if not isinstance(flow, DummyFlow):
+            raise ValueError("Invalid flow")
+
+        model = DummyModel()
+        model.defaults = initialize_with_defaults
+        model.strict_version = strict_version
+        return model
+
+    def model_to_flow(self, model):
+        if not isinstance(model, DummyModel):
+            raise ValueError("Invalid model")
+        return DummyFlow()
+
+    def get_version_information(self):
+        return ["dummy==1.0"]
+
+    def create_setup_string(self, model):
+        return "DummyModel()"
+
+    def is_estimator(self, model):
+        return isinstance(model, DummyModel)
+
+    def seed_model(self, model, seed):
+        model.seed = seed
+        return model
+
+    def _run_model_on_fold(
+        self,
+        model,
+        task,
+        X_train,
+        rep_no,
+        fold_no,
+        y_train=None,
+        X_test=None,
+    ):
+        preds = np.zeros(len(X_train))
+        probs = None
+        measures = OrderedDict()
+        trace = None
+        return preds, probs, measures, trace
+
+    def obtain_parameter_values(self, flow, model=None):
+        return []
+
+    def check_if_model_fitted(self, model):
+        return False
+
+    def instantiate_model_from_hpo_class(self, model, trace_iteration):
+        return DummyModel()
+
 
 
 class TestInit(openml.testing.TestBase):
-    def setUp(self):
-        super().setUp()
-        _unregister()
 
     def test_get_extension_by_flow(self):
-        assert get_extension_by_flow(DummyFlow()) is None
-        with pytest.raises(ValueError, match="No extension registered which can handle flow:"):
-            get_extension_by_flow(DummyFlow(), raise_if_no_extension=True)
-        register_extension(DummyExtension1)
-        assert isinstance(get_extension_by_flow(DummyFlow()), DummyExtension1)
-        register_extension(DummyExtension2)
-        assert isinstance(get_extension_by_flow(DummyFlow()), DummyExtension1)
-        register_extension(DummyExtension1)
-        with pytest.raises(
-            ValueError, match="Multiple extensions registered which can handle flow:"
-        ):
-            get_extension_by_flow(DummyFlow())
+            # We replace the global list with a new empty list [] ONLY for this block
+            with patch("openml.extensions.extensions", []):
+                assert get_extension_by_flow(DummyFlow()) is None
+                
+                with pytest.raises(ValueError, match="No extension registered which can handle flow:"):
+                    get_extension_by_flow(DummyFlow(), raise_if_no_extension=True)
+                
+                register_extension(DummyExtension1)
+                assert isinstance(get_extension_by_flow(DummyFlow()), DummyExtension1)
+                
+                register_extension(DummyExtension2)
+                assert isinstance(get_extension_by_flow(DummyFlow()), DummyExtension1)
+                
+                register_extension(DummyExtension1)
+                with pytest.raises(
+                    ValueError, match="Multiple extensions registered which can handle flow:"
+                ):
+                    get_extension_by_flow(DummyFlow())
 
     def test_get_extension_by_model(self):
-        assert get_extension_by_model(DummyModel()) is None
-        with pytest.raises(ValueError, match="No extension registered which can handle model:"):
-            get_extension_by_model(DummyModel(), raise_if_no_extension=True)
-        register_extension(DummyExtension1)
-        assert isinstance(get_extension_by_model(DummyModel()), DummyExtension1)
-        register_extension(DummyExtension2)
-        assert isinstance(get_extension_by_model(DummyModel()), DummyExtension1)
-        register_extension(DummyExtension1)
-        with pytest.raises(
-            ValueError, match="Multiple extensions registered which can handle model:"
-        ):
-            get_extension_by_model(DummyModel())
+        # Again, we start with a fresh empty list automatically
+        with patch("openml.extensions.extensions", []):
+            assert get_extension_by_model(DummyModel()) is None
+            
+            with pytest.raises(ValueError, match="No extension registered which can handle model:"):
+                get_extension_by_model(DummyModel(), raise_if_no_extension=True)
+            
+            register_extension(DummyExtension1)
+            assert isinstance(get_extension_by_model(DummyModel()), DummyExtension1)
+            
+            register_extension(DummyExtension2)
+            assert isinstance(get_extension_by_model(DummyModel()), DummyExtension1)
+            
+            register_extension(DummyExtension1)
+            with pytest.raises(
+                ValueError, match="Multiple extensions registered which can handle model:"
+            ):
+                get_extension_by_model(DummyModel())
+
+
+def test_flow_to_model_with_defaults():
+    """Test flow_to_model with initialize_with_defaults=True."""
+    ext = DummyExtension()
+    flow = DummyFlow()
+
+    model = ext.flow_to_model(flow, initialize_with_defaults=True)
+
+    assert isinstance(model, DummyModel)
+    assert model.defaults is True
+
+def test_flow_to_model_strict_version():
+    """Test flow_to_model with strict_version parameter."""
+    ext = DummyExtension()
+    flow = DummyFlow()
+
+    model_strict = ext.flow_to_model(flow, strict_version=True)
+    model_non_strict = ext.flow_to_model(flow, strict_version=False)
+
+    assert isinstance(model_strict, DummyModel)
+    assert model_strict.strict_version is True
+
+    assert isinstance(model_non_strict, DummyModel)
+    assert model_non_strict.strict_version is False
+
+def test_model_to_flow_conversion():
+    """Test converting a model back to flow representation."""
+    ext = DummyExtension()
+    model = DummyModel()
+
+    flow = ext.model_to_flow(model)
+
+    assert isinstance(flow, DummyFlow)
+
+
+def test_invalid_flow_raises_error():
+    """Test that invalid flow raises appropriate error."""
+    class InvalidFlow:
+        pass
+
+    ext = DummyExtension()
+    flow = InvalidFlow()
+
+    with pytest.raises(ValueError, match="Invalid flow"):
+        ext.flow_to_model(flow)
+
+
+@patch("openml.extensions.extensions", [])
+def test_extension_not_found_error_message():
+    """Test error message contains helpful information."""
+    class UnknownModel:
+        pass
+
+    with pytest.raises(ValueError, match="No extension registered"):
+        get_extension_by_model(UnknownModel(), raise_if_no_extension=True)
+
+ 
+def test_register_same_extension_twice():
+    """Test behavior when registering same extension twice."""
+    # Using a context manager here to isolate the list
+    with patch("openml.extensions.extensions", []):
+        register_extension(DummyExtension)
+        register_extension(DummyExtension)
+
+        matches = [
+            ext for ext in openml.extensions.extensions
+            if ext is DummyExtension
+        ]
+        assert len(matches) == 2
+
+
+@patch("openml.extensions.extensions", [])
+def test_extension_priority_order():
+    """Test that extensions are checked in registration order."""    
+    class DummyExtensionA(DummyExtension):
+        pass
+    class DummyExtensionB(DummyExtension):
+        pass
+
+    register_extension(DummyExtensionA)
+    register_extension(DummyExtensionB)
+
+    assert openml.extensions.extensions[0] is DummyExtensionA
+    assert openml.extensions.extensions[1] is DummyExtensionB