fix: update gemini response schema key for JSON format handling

Author: jiapan

livekit-plugins/livekit-plugins-google/livekit/plugins/google/llm.py

@@ -261,7 +261,7 @@ def chat(
                 extra["tool_config"] = gemini_tool_choice
 
         if is_given(response_format):
-            extra["response_schema"] = to_response_format(response_format)  # type: ignore
+            extra["response_json_schema"] = to_response_format(response_format)  # type: ignore
             extra["response_mime_type"] = "application/json"
 
         if is_given(self._opts.temperature):