Revert using v2 prompts endpoint for chat requests

What does this MR do and why?

Revert using v2 prompts endpoint for chat requests

In !244956 (merged) we introduced the v2/prompts endpoint for Classic Chat requests, in hopes that the v1 endpoint could be completely retired. But the chat code is incompatible with the non-plain-strings response structure of the v2 endpoint. For now, let's exclude Chat requests from using the v2 endpoint to avoid broken behavior until it's addressed.

Related to #552131

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