fix(fireworks): drop reasoning history blocks (#39973)

Fixed Fireworks requests failing after switching from a model that
stores reasoning blocks in conversation history.

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Users switching from an OpenAI Responses model to Fireworks could
receive a 400 because canonical `reasoning` blocks remained in
conversation history. `ChatFireworks` now drops those provider-specific
blocks before serializing Chat Completions requests, matching its
handling of other unsupported reasoning formats.

Made by [Open
SWE](https://openswe.vercel.app/agents/15bd8573-dbbf-55f8-8f22-d5295ec6de11)

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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>