fix(#5818): Force to use string serializer for deepseek function calling

[xingyaoww]

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Force to use string serializer for deepseek function calling, otherwise we will end up with the error in #5818.

Running an evaluation now with Deepseek w/ function calling..

Still use our non-fncall converter for Deepseek - see results here: https://x.com/xingyaow_/status/1872375832041103638

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[enyst]

Looking good to me!

I knew this moment will come, when the string/list choice will not depend only on our attributes...😅 I think this might be something to fix in litellm if indeed the list format is sent but not supported for deepseek.

Just left a comment to keep our changes minimal.