fix: Clear restriction service singleton in o3-pro test setup

The test was failing when run in the full test suite because the
ModelRestrictionService singleton persisted restrictions from previous
tests. Specifically, test_fallback_with_shorthand_restrictions sets
OPENAI_ALLOWED_MODELS="mini" which blocked o3-pro.

Added utils.model_restrictions._restriction_service = None to ensure
the test starts with clean restriction state.

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

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

tests/test_o3_pro_output_text_fix.py

@@ -34,6 +34,11 @@ class TestO3ProOutputTextFix:
 
     def setup_method(self):
         """Set up the test by ensuring clean registry state."""
+        # Clear the restriction service singleton to ensure clean state
+        import utils.model_restrictions
+
+        utils.model_restrictions._restriction_service = None
+
         # Use the new public API for registry cleanup
         ModelProviderRegistry.reset_for_testing()
         # Provider registration is now handled by inject_transport helper