Enable (optional) concurrent execution of function calls in get_function_response_parts_async

[marinburazin]

The problem:

• Relevant only for async client with enabled automatic function calling
• Currently, if a LLM returns two or more functions for execution, these will be executed sequentially.
• Considering these can often be blocking external API calls, the sequential wait for all of them to complete before returning can quickly mount, resulting in increased latency.

Proposed solution

• Expand AutomaticFunctionCallingConfig and AutomaticFunctionCallingConfigDict with an optional parameter (e.g. run_concurrently), which defaults to None if not provided.
• Expand get_function_response_parts_async so it makes use of asyncio.gather() if run_concurrently is set to True .

Client-side example

• In the end, it would look something like this on the client side:

    response = await client.aio.models.generate_content(
        model='gemini-2.5-flash',
        contents='What is weather and traffic situation in Boston?',
        config=types.GenerateContentConfig(
            tools=[get_current_weather, get_current_traffic],
            automatic_function_calling=types.AutomaticFunctionCallingConfig(run_concurrently=True)
        ),
    )