feat: Return request handlers from decorator methods to allow further decoration (#934)

### Description
Currently, wrapping a handler with a decorator nullifies the function
because it returns `None` [in the
code](https://github.com/apify/crawlee-python/blob/28579996e454e19dd69ed6ee09e4293d4e056a6e/src/crawlee/router.py#L44-L45).
See the "None" popup over here:

![image](https://github.com/user-attachments/assets/96df44c5-3ceb-4145-9bdd-f697bdf2da3d)

By having the router.handler function return the original input handler,
we can now reuse the handler function again in other places as it's not
set to `None` anymore. This allows for invoking the original function
even after applying the decorator and for adding multiple decorators to
the same handler function. The latter for example allows for using the
same handler function for multiple labels (a use case I ran into).

### Issues
Not applicable.

### Testing

- [x] Added a test to ensure handler is not nullified
- [x] Added a test for multiple labels assigned to a single handler

### Checklist

- [x] CI passed

Please let me know your thoughts! 🙌

Author: mylank

src/crawlee/router.py

@@ -33,16 +33,19 @@ def default_handler(self: Router, handler: RequestHandler[TCrawlingContext]) ->
 
         return handler
 
-    def handler(self, label: str) -> Callable[[RequestHandler[TCrawlingContext]], None]:
+    def handler(
+        self, label: str
+    ) -> Callable[[RequestHandler[TCrawlingContext]], Callable[[TCrawlingContext], Awaitable]]:
         """A decorator used to register a label-based handler.
 
         The registered will be invoked only for requests with the exact same label.
         """
         if label in self._handlers_by_label:
             raise RuntimeError(f'A handler for label `{label}` is already registered')
 
-        def wrapper(handler: Callable[[TCrawlingContext], Awaitable]) -> None:
+        def wrapper(handler: Callable[[TCrawlingContext], Awaitable]) -> Callable[[TCrawlingContext], Awaitable]:
             self._handlers_by_label[label] = handler
+            return handler
 
         return wrapper
 

tests/unit/test_router.py

@@ -89,3 +89,29 @@ async def default_handler(_context: MockContext) -> None:
     mock_default_handler.assert_not_called()
     mock_handler_a.assert_not_called()
     mock_handler_b.assert_called()
+
+
+async def test_router_handler_not_nullified() -> None:
+    router = Router[MockContext]()
+
+    @router.handler('A')
+    async def handler_a(_context: MockContext) -> None:
+        pass
+
+    assert handler_a is not None
+
+
+async def test_router_multi_labelled_handler() -> None:
+    router = Router[MockContext]()
+    mock_handler = Mock()
+
+    @router.handler('A')
+    @router.handler('B')
+    async def handler(_context: MockContext) -> None:
+        mock_handler(_context.request.label)
+
+    await router(MockContext(label='A'))
+    mock_handler.assert_called_with('A')
+    await router(MockContext(label='B'))
+    mock_handler.assert_called_with('B')
+    assert mock_handler.call_count == 2