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 [email protected] Assisted-By: Claude Fable 5 [email protected]

Co-authored-by: tarruda [email protected]