refactor: update authentication handling and model configuration

- Enhanced authentication method validation in `auth.ts` and `auth.test.ts`.
- Introduced new model provider configuration logic
- Updated environment variable handling for various auth types.
- Removed deprecated utility functions and tests related to fallback mechanisms.

packages/cli/src/ui/hooks/useGeminiStream.ts

@@ -912,7 +912,7 @@ export const useGeminiStream = (
       // Reset quota error flag when starting a new query (not a continuation)
       if (!options?.isContinuation) {
         setModelSwitchedFromQuotaError(false);
-        config.setQuotaErrorOccurred(false);
+        // No quota-error / fallback routing mechanism currently; keep state minimal.
       }
 
       abortControllerRef.current = new AbortController();