chat: trim messages sent to StepFun parser (fixes long reasoning loops) (#25238)

chat: trim messages sent to StepFun parser (fixes long reasoning loops) (#25238)

* chat: trim messages sent to StepFun parser (fixes long reasoning loops)

* add regression test; remove duplicate template

* chat: trim StepFun content parts before rendering

The StepFun trim workaround ran on the already-rendered messages, where
typed content parts have been concatenated into a single string, so the
per-part whitespace could no longer be reached. Move the trim ahead of
rendering and apply it to content_parts text as well as the string
content and reasoning_content. Adds a content-parts regression test.

Co-Authored-By: Piotr Wilkin <redacted>
Assisted-By: Claude Fable 5 <redacted>
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Co-authored-by: tarruda <redacted>