feat: Implement code review improvements from gemini-2.5-pro analysis

✨ Key improvements:
• Added public reset_for_testing() method to registry for clean test state management
• Updated test setup/teardown to use new public API instead of private attributes
• Enhanced inject_transport helper to ensure OpenAI provider registration
• Migrated additional test files to use inject_transport pattern
• Reduced code duplication by ~30 lines across test files

🔧 Technical details:
• transport_helpers.py: Always register OpenAI provider for transport tests
• test_o3_pro_output_text_fix.py: Use reset_for_testing() API, remove redundant registration
• test_o3_pro_fixture_bisect.py: Migrate all 4 test methods to inject_transport
• test_o3_pro_simplified.py: Migrate both test methods to inject_transport
• providers/registry.py: Add reset_for_testing() public method

✅ Quality assurance:
• All 7 o3-pro tests pass with new helper pattern
• No regression in test isolation or provider state management
• Improved maintainability through centralized transport injection
• Follows single responsibility principle with focused helper function

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

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

providers/registry.py

@@ -441,6 +441,17 @@ class ModelProviderRegistry:
         instance = cls()
         instance._initialized_providers.clear()
 
+    @classmethod
+    def reset_for_testing(cls) -> None:
+        """Reset the registry to a clean state for testing.
+
+        This provides a safe, public API for tests to clean up registry state
+        without directly manipulating private attributes.
+        """
+        cls._instance = None
+        if hasattr(cls, "_providers"):
+            cls._providers = {}
+
     @classmethod
     def unregister_provider(cls, provider_type: ProviderType) -> None:
         """Unregister a provider (mainly for testing)."""