fix: Resolve test isolation issues for o3-pro test

- Fixed test_fallback_with_shorthand_restrictions to clear restriction
  service singleton in finally block, preventing state leakage
- Updated o3-pro test to use @patch.dict for OPENAI_ALLOWED_MODELS,
  following standard pattern and allowing both o3-pro and o3-pro-2025-06-10
- Removed invalid cassette file that had wrong request content

The test now passes in both isolated and full suite runs.

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

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

tests/transport_helpers.py

@@ -22,11 +22,10 @@ def inject_transport(monkeypatch, cassette_path: str):
         transport = inject_transport(monkeypatch, "path/to/cassette.json")
     """
     # Ensure OpenAI provider is registered - always needed for transport injection
-    import os
     from providers.base import ProviderType
     from providers.openai_provider import OpenAIModelProvider
     from providers.registry import ModelProviderRegistry
-    
+
     # Always register OpenAI provider for transport tests (API key might be dummy)
     ModelProviderRegistry.register_provider(ProviderType.OPENAI, OpenAIModelProvider)