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/security/test_security_config.py

@@ -67,9 +67,6 @@ def test_security_config_from_dict():
     }
 
     # Create a config dict with just the fingerprint
-    config_dict = {
-        "fingerprint": fingerprint_dict
-    }
 
     # Create config manually since from_dict has a specific implementation
     config = SecurityConfig()
@@ -115,4 +112,4 @@ def test_security_config_json_serialization():
     new_config.fingerprint = new_fingerprint
 
     # Check the new config has the same fingerprint metadata
-    assert new_config.fingerprint.metadata == {"version": "1.0"}
+    assert new_config.fingerprint.metadata == {"version": "1.0"}