Fix parallel_tool_calls when False

[apeccaud]

Currently, when we set parallel_tool_calls=False in the model_settings, the responses API is called with parallel_tool_calls == NotGiven, which defaults to true on the Response API side (https://platform.openai.com/docs/api-reference/responses/create#responses-create-parallel_tool_calls).

This PR attempts to fix that.

agent = Agent(
    ...,
    model_settings=ModelSettings(
        parallel_tool_calls=False,
    ),
)

[rm-openai]

Thanks for fixing!