fix: update gemini response schema key for JSON format handling

[Panmax]

According to the official documentation, it should be response_json_schema

https://ai.google.dev/gemini-api/docs/structured-output

[CLAassistant]

All committers have signed the CLA.

[longcw]

@Panmax should we also return the json schema instead of _GeminiJsonSchema(schema).simplify() from to_response_format, or it's still required? https://github.com/livekit/agents/blob/[email protected]/livekit-plugins/livekit-plugins-google/livekit/plugins/google/utils.py#L152