Fix lint issues and update agent.execute_task for recursion depth

- Remove unused imports and variables in test files
- Replace bare except with specific exception in structured_output_converter.py
- Fix None comparison in llm_test.py
- Update agent.execute_task to accept recursion_depth parameter

Resolves all remaining lint issues for A2A protocol implementation.

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

tests/test_lite_agent.py

@@ -1,4 +1,3 @@
-import asyncio
 from typing import cast
 from unittest.mock import Mock
 
@@ -313,5 +312,5 @@ def test_sets_parent_flow_when_inside_flow():
             nonlocal captured_agent
             captured_agent = source
 
-        result = flow.kickoff()
+        flow.kickoff()
         assert captured_agent.parent_flow is flow