fix: Resolve o3-pro response parsing and test execution issues

- Fix lint errors: trailing whitespace and deprecated typing imports
- Update test mock for o3-pro response format (output.content[] → output_text)
- Implement robust test isolation with monkeypatch fixture
- Clear provider registry cache to prevent test interference
- Ensure o3-pro tests pass in both individual and full suite execution

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/conftest.py

@@ -93,7 +93,7 @@ def pytest_collection_modifyitems(session, config, items):
         if item.get_closest_marker("no_mock_provider"):
             config._needs_dummy_keys = False
             break
-    
+
     # Set dummy keys only if no test needs real keys
     if config._needs_dummy_keys:
         _set_dummy_keys_if_missing()