fix: address Copilot review on OpenAI-compatible providers (#5042) (#5089)

- Delegate supports_function_calling() to parent (handles o1 models via OpenRouter)
- Guard empty env vars in base_url resolution
- Fix misleading comment about model validation rules
- Remove unused MagicMock import
- Use 'is not None' for env var restoration in tests

Co-authored-by: Joao Moura <joao@crewai.com>

lib/crewai/tests/llms/openai_compatible/test_openai_compatible.py

@@ -1,7 +1,7 @@
 """Tests for OpenAI-compatible providers."""
 
 import os
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 
 import pytest
 
@@ -133,7 +133,7 @@ class TestOpenAICompatibleCompletion:
             with pytest.raises(ValueError, match="API key required"):
                 OpenAICompatibleCompletion(model="deepseek-chat", provider="deepseek")
         finally:
-            if original:
+            if original is not None:
                 os.environ[env_key] = original
 
     def test_api_key_from_env(self):