feat: Enhance A2A integration with modular architecture

Address review comments from João:
- Add TaskInfo dataclass for enhanced task management with status tracking
- Add ServerConfig dataclass for improved server configuration management
- Add custom exception classes (A2AServerError, TransportError, ExecutionError)
- Refactor code to use modular components for better maintainability
- Update output conversion to handle JSON data types properly
- Improve error handling with granular exception types
- All tests pass (30 passed, 6 skipped)

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

tests/a2a/test_server.py

@@ -95,7 +95,8 @@ class TestA2AServer:
     
     def test_create_a2a_app_fastapi(self, mock_agent_executor):
         """Test creating A2A app with FastAPI transport raises error."""
-        with pytest.raises(ValueError, match="FastAPI transport is not available"):
+        from crewai.a2a.exceptions import TransportError
+        with pytest.raises(TransportError, match="FastAPI transport is not available"):
             create_a2a_app(
                 mock_agent_executor,
                 transport="fastapi",