refactor(cli): Finalize OpenAI Codex Integration with OAuth

- Enhanced Codex model discovery by fetching available models from the API, with fallback to local cache and defaults.
- Updated the context compressor's summary target tokens to 2500 for improved performance.
- Added external credential detection for Codex CLI to streamline authentication.
- Refactored various components to ensure consistent handling of authentication and model selection across the application.

tests/test_codex_execution_paths.py

@@ -149,6 +149,11 @@ def test_gateway_run_agent_codex_path_handles_internal_401_refresh(monkeypatch):
     runner._prefill_messages = []
     runner._reasoning_config = None
     runner._running_agents = {}
+    from unittest.mock import MagicMock, AsyncMock
+    runner.hooks = MagicMock()
+    runner.hooks.emit = AsyncMock()
+    runner.hooks.loaded_hooks = []
+    runner._session_db = None
 
     source = SessionSource(
         platform=Platform.LOCAL,