docs: align endpoint-test docstring + module-doc reference (#105 PR 2a)

Two comment-only cleanups; no behavior change.

(1) tests/test_binding_validation.py:431 — TestEndpointTypeMismatch::
test_edge_endpoint_type_does_not_match_referenced_entity_key
docstring previously said two ENDPOINT_TYPE_MISMATCH entries are
expected. The implementation suppresses the physical (2) check in
edge-only-drift cases as of commit 2342165, so only one entry is
expected now. Docstring updated to match, with a pointer to
TestEndpointPhysicalCrossTableCheck for the node-drifted case
where (2) genuinely adds information.

(2) src/bigquery_agent_analytics/binding_validation.py:23 — module
docstring referenced bigquery_agent_analytics.graph_validation
.validate_extracted_graph as if it existed. That symbol lands with
issue #76 and is not on this PR. Reworded to plain text:
'Different from the planned extracted-graph validator in issue #76',
with an explicit warning that no graph_validation module exists yet
so callers do not try to import it.

25/25 unit tests pass.

Author: caohy1988

src/bigquery_agent_analytics/binding_validation.py

@@ -20,10 +20,12 @@
 most common authoring error (binding YAML drifted out of sync with
 physical tables) before extraction wastes ``AI.GENERATE`` tokens.
 
-Different from :func:`bigquery_agent_analytics.graph_validation.
-validate_extracted_graph` (issue #76): that one validates extracted
-graph output against the spec. This one validates the binding against
-the live BigQuery schemas.
+Different from the planned extracted-graph validator in issue #76:
+that one will validate extracted graph output against the spec
+(post-extraction, pre-materialization). This one validates the
+binding against the live BigQuery schemas (pre-extraction). #76 has
+not landed yet, so do not import any symbol from a
+``graph_validation`` module — it does not exist on this PR.
 
 Usage::
 

tests/test_binding_validation.py

@@ -429,12 +429,14 @@ def test_edge_endpoint_type_does_not_match_referenced_entity_key(
       self,
   ):
     """Edge endpoint type disagrees with the referenced node's
-    ontology key type. Two ENDPOINT_TYPE_MISMATCH entries are
-    expected — one from the spec-level (1) check (edge BQ type vs
-    ontology-derived expected SDK type) and one from the physical
-    cross-table (2) check (edge BQ type vs node table's actual key
-    BQ type). Both are correct and complementary; users see both
-    descriptions and can act on either."""
+    ontology key type, but the node table itself is on-spec. One
+    ENDPOINT_TYPE_MISMATCH entry is expected — the spec-level (1)
+    check (edge BQ type vs ontology-derived expected SDK type).
+    The physical cross-table (2) check is suppressed because
+    emitting it would double-report the same expected/observed pair
+    with different detail wording. The node-drifted case where (2)
+    adds genuinely new information is covered by
+    TestEndpointPhysicalCrossTableCheck."""
     from bigquery_agent_analytics.binding_validation import FailureCode
     from bigquery_agent_analytics.binding_validation import validate_binding_against_bigquery