chore(core): refactor model resolution and cleanup fallback logic (#15228)

packages/core/src/fallback/handler.ts

@@ -25,18 +25,6 @@ export async function handleFallback(
   failedModel: string,
   authType?: string,
   error?: unknown,
-): Promise<string | boolean | null> {
-  return handlePolicyDrivenFallback(config, failedModel, authType, error);
-}
-
-/**
- * New fallback logic using the ModelAvailabilityService
- */
-async function handlePolicyDrivenFallback(
-  config: Config,
-  failedModel: string,
-  authType?: string,
-  error?: unknown,
 ): Promise<string | boolean | null> {
   if (authType !== AuthType.LOGIN_WITH_GOOGLE) {
     return null;