[Fix] Serve drops some attributes at serialization

[remi-or]

This PR fixes an issue in serve.
In chat_completion.py, we create a subclass of ChatCompletionMessage in which reasoning_content is a field. When this class is serialized, the field is lost because pydantic v2 uses the serializer from the annotated class (the original ChatCompletionMessage that was subclassed) rather than the runtime class (the subclass that we instantiate). Since the parent class, and its serializer, don't know about this subclass's attribute, it is lost at serialisation.

This PR fixes this by removing the subclass and rather passing reasoning_content as an extra field, which is allowed by the base class and serialized as well.
This fixes 6 tests in test_serve.py

Since I am not familiar with pydantic, this may not be the best way to resolve this. On the off-chance it is, happy to help.

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[SunMarc]

Thanks, I've fixed this issue on this PR but still waiting for @Rocketknight1 PR to be merged. #46267
Still the solution here seems a bit more elegant than what i did so I will update my PR later on.