feat(livekit-plugins-google): structured responses for Live API tool calls

[vsomayaji]

LiveKit automatically converts the result of a tool call to a string (reference), and this string was being returned in the Live API FunctionResponse.response as output. So, for example:

@function_tool()
async def lookup_weather(self, context: RunContext, location: str) -> dict[str, Any]:
    return {"weather": "sunny", "temperature_f": 70}

Would get returned to the Live API as:

{
  "output": "{'weather': 'sunny', 'temperature_f': 70}"
}

However, our Live API integration requires returning the actual result, not the string representation.

With this change, we:

1. Do not return a FunctionResponse if the tool call did not return anything.
2. Set FunctionResponse.response to the actual result by calling ast.literal_eval on the string representation.
3. Set FunctionResponse.scheduling to SILENT to let the model decide when to use the result (reference).