Fix #980 Chat Completions: fails with function name for tool_choice parameter w/ streaming enabled

[seratch]

This pull request resolves #980

simple code snippet to do tests:

import asyncio
from openai.types.responses import ResponseTextDeltaEvent
from agents import Agent, ModelSettings, Runner, function_tool, set_default_openai_api

set_default_openai_api("chat_completions")

@function_tool
def say_hello():
    print("Hello, world!")
    return "Hello, world!"

async def main():
    agent = Agent(
        name="Joker",
        instructions="You are a helpful assistant.",
        model_settings=ModelSettings(tool_choice="say_hello"),
        tools=[say_hello],
    )
    result = Runner.run_streamed(agent, input="Please tell me 5 jokes.")
    async for event in result.stream_events():
        if event.type == "raw_response_event" and isinstance(event.data, ResponseTextDeltaEvent):
            print(event.data.delta, end="", flush=True)

if __name__ == "__main__":
    asyncio.run(main())

[seratch]

The essential changes here are 1) use Converter.convert_tool_choice 2) cover responses_tool_choice is None pattern. Added more detailed comments about why it's necessary but happy to adjust the style if necessary