improvement(providers): harden OpenAI-compatible providers + add tests #4796

Bugbot Review

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Final Result: Bugbot completed review - no issues found! ✅

Request ID: serverGenReqId_b7b5b0dd-af25-41ec-a96e-1d12fa7d08a6

✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 0be3ca2. Configure here.}