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/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. "