feat: add transport negotiation and content type handling

- add transport negotiation logic with fallback support
- add content type parser and encoder utilities
- add transport configuration models (client and server)
- add transport types and enums
- enhance config with transport settings
- add negotiation events for transport and content type

lib/crewai/tests/a2a/utils/test_task.py

@@ -43,6 +43,7 @@ def mock_context() -> MagicMock:
     context.context_id = "test-context-456"
     context.get_user_input.return_value = "Test user message"
     context.message = MagicMock(spec=Message)
+    context.message.parts = []
     context.current_task = None
     return context