refactor: Centralize and improve model fallback handling (#7634)

packages/core/src/config/config.ts

@@ -52,6 +52,7 @@ import type { FileSystemService } from '../services/fileSystemService.js';
 import { StandardFileSystemService } from '../services/fileSystemService.js';
 import { logCliConfiguration, logIdeConnection } from '../telemetry/loggers.js';
 import { IdeConnectionEvent, IdeConnectionType } from '../telemetry/types.js';
+import type { FallbackModelHandler } from '../fallback/types.js';
 
 // Re-export OAuth config type
 export type { MCPOAuthConfig, AnyToolInvocation };
@@ -157,12 +158,6 @@ export interface SandboxConfig {
   image: string;
 }
 
-export type FlashFallbackHandler = (
-  currentModel: string,
-  fallbackModel: string,
-  error?: unknown,
-) => Promise<boolean | string | null>;
-
 export interface ConfigParameters {
   sessionId: string;
   embeddingModel?: string;
@@ -281,7 +276,7 @@ export class Config {
     name: string;
     extensionName: string;
   }>;
-  flashFallbackHandler?: FlashFallbackHandler;
+  fallbackModelHandler?: FallbackModelHandler;
   private quotaErrorOccurred: boolean = false;
   private readonly summarizeToolOutput:
     | Record<string, SummarizeToolOutputSettings>
@@ -490,8 +485,8 @@ export class Config {
     this.inFallbackMode = active;
   }
 
-  setFlashFallbackHandler(handler: FlashFallbackHandler): void {
-    this.flashFallbackHandler = handler;
+  setFallbackModelHandler(handler: FallbackModelHandler): void {
+    this.fallbackModelHandler = handler;
   }
 
   getMaxSessionTurns(): number {