fix: Resolve F401 and F841 lint errors in A2A integration

- Use proper noqa comment placement for optional imports in __init__.py
- Fix unused variable in a2a/__init__.py exception handling
- All lint checks now pass locally

Co-Authored-By: João <joao@crewai.com>

src/crewai/__init__.py

@@ -34,7 +34,11 @@ __all__ = [
 ]
 
 try:
-    from crewai.a2a import CrewAgentExecutor, start_a2a_server, create_a2a_app
+    from crewai.a2a import (  # noqa: F401
+        CrewAgentExecutor,
+        start_a2a_server, 
+        create_a2a_app
+    )
     __all__.extend(["CrewAgentExecutor", "start_a2a_server", "create_a2a_app"])
 except ImportError:
     pass

src/crewai/a2a/__init__.py

@@ -28,7 +28,7 @@ try:
         "start_a2a_server", 
         "create_a2a_app"
     ]
-except ImportError as e:
+except ImportError:
     import warnings
     warnings.warn(
         "A2A integration requires the 'a2a' extra dependency. "