fix(core): propagate BeforeModel hook model override end-to-end (#24784)

Signed-off-by: krishdef7 <gargkrish06@gmail.com>
Co-authored-by: Sandy Tao <sandytao520@icloud.com>

Author: krishdef7

packages/core/src/core/geminiChat.ts

@@ -597,6 +597,21 @@ export class GeminiChat {
           );
         }
 
+        if (beforeModelResult.modifiedModel) {
+          modelToUse = resolveModel(
+            beforeModelResult.modifiedModel,
+            useGemini3_1,
+            useGemini3_1FlashLite,
+            false,
+            hasAccessToPreview,
+            this.context.config,
+          );
+          lastModelToUse = modelToUse;
+          // Re-evaluate contentsToUse based on the new model's feature support
+          contentsToUse = supportsModernFeatures(modelToUse)
+            ? [...contentsForPreviewModel]
+            : [...requestContents];
+        }
         if (beforeModelResult.modifiedConfig) {
           Object.assign(config, beforeModelResult.modifiedConfig);
         }

packages/core/src/hooks/hookSystem.ts

@@ -48,6 +48,8 @@ export interface BeforeModelHookResult {
   reason?: string;
   /** Synthetic response to return instead of calling the model (if blocked) */
   syntheticResponse?: GenerateContentResponse;
+  /** Modified model override (if not blocked) */
+  modifiedModel?: string;
   /** Modified config (if not blocked) */
   modifiedConfig?: GenerateContentConfig;
   /** Modified contents (if not blocked) */
@@ -292,6 +294,7 @@ export class HookSystem {
           beforeModelOutput.applyLLMRequestModifications(llmRequest);
         return {
           blocked: false,
+          modifiedModel: modifiedRequest?.model,
           modifiedConfig: modifiedRequest?.config,
           modifiedContents: modifiedRequest?.contents,
         };