fix(ai): switch grounding tool from GoogleSearchRetrieval to GoogleSearch

Gemini API no longer supports googleSearchRetrieval; requests fail with
INVALID_ARGUMENT. Replace with the google_search tool as required.

backend/internal/pkg/ai/gemini.go

@@ -237,7 +237,7 @@ func (p *GeminiProvider) Chat(ctx context.Context, req *ChatRequest) (*ChatRespo
 	// Google Search grounding
 	if req.Grounded {
 		cfg.Tools = []*genai.Tool{
-			{GoogleSearchRetrieval: &genai.GoogleSearchRetrieval{}},
+			{GoogleSearch: &genai.GoogleSearch{}},
 		}
 	}