Served models handle with nested content

[jakeret]

What does this PR do?

This PR fixes an issue in the transformers serve functionality where the server could not handle chat messages with nested content (e.g., when the content field in a message was a list instead of a string or dictionary). The unhandled TypeError caused the server to crash instead of processing the request or returning a meaningful error message.

Fixes #39791

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Who can review?

Anyone in the community is free to review the PR once the tests have passed.

[gante]

@jakeret thank you for opening the PR (and sorry for not noticing it before opening #39829 !)

I think #39829 cover more cases (a list with multiple items in content), but let me know if you think it misses some functionality :)

[jakeret]

@gante no worries. In my tests with gradio I didn't encounter the case where the list contained more than one item. Thats why I only fetched the first item.
Could it be that #39829 now misses one of the original cases where content=message["content"]["text"] ?