Fix not using flash for next speaker check (#4016)

packages/core/src/utils/nextSpeakerChecker.ts

@@ -5,6 +5,7 @@
  */
 
 import { Content, SchemaUnion, Type } from '@google/genai';
+import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
 import { GeminiClient } from '../core/client.js';
 import { GeminiChat } from '../core/geminiChat.js';
 import { isFunctionResponse } from './messageInspectors.js';
@@ -131,6 +132,7 @@ export async function checkNextSpeaker(
       contents,
       RESPONSE_SCHEMA,
       abortSignal,
+      DEFAULT_GEMINI_FLASH_MODEL,
     )) as unknown as NextSpeakerResponse;
 
     if (