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_fingerprint.py

@@ -5,7 +5,6 @@ import uuid
 from datetime import datetime, timedelta
 
 import pytest
-from pydantic import ValidationError
 
 from crewai.security import Fingerprint
 
@@ -223,7 +222,7 @@ def test_invalid_uuid_str():
 
     # But this will raise an exception when we try to access the uuid property
     with pytest.raises(ValueError):
-        uuid_obj = fingerprint.uuid
+        fingerprint.uuid
 
 
 def test_fingerprint_metadata_mutation():
@@ -260,4 +259,4 @@ def test_fingerprint_metadata_mutation():
 
     # Ensure immutable fields remain unchanged
     assert fingerprint.uuid_str == uuid_str
-    assert fingerprint.created_at == created_at
+    assert fingerprint.created_at == created_at