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>

src/crewai/agent.py

@@ -225,6 +225,7 @@ class Agent(BaseAgent):
         task: Task,
         context: Optional[str] = None,
         tools: Optional[List[BaseTool]] = None,
+        recursion_depth: int = 0,
     ) -> str:
         """Execute a task with the agent.
 

src/crewai/agents/agent_adapters/langgraph/langgraph_adapter.py

@@ -1,4 +1,4 @@
-from typing import Any, AsyncIterable, Dict, List, Optional
+from typing import Any, Dict, List, Optional
 
 from pydantic import Field, PrivateAttr
 
@@ -22,7 +22,6 @@ from crewai.utilities.events.agent_events import (
 )
 
 try:
-    from langchain_core.messages import ToolMessage
     from langgraph.checkpoint.memory import MemorySaver
     from langgraph.prebuilt import create_react_agent
 

src/crewai/agents/agent_adapters/langgraph/structured_output_converter.py

@@ -74,7 +74,7 @@ The output should be raw JSON that exactly matches the specified schema.
                         # Validate it's proper JSON
                         json.loads(extracted)
                         return extracted
-                    except:
+                    except json.JSONDecodeError:
                         pass
 
         return result