Preserve OpenAI-compatible replay reasoning when the selected custom or self-hosted model already has reasoning metadata enabled.
The transcript policy now treats existing model metadata as the replay contract instead of requiring a new provider config knob, and the OpenAI-compatible serializer preserves reasoning_content for those routes while keeping stock OpenAI, Gemma 4, and known non-replayable OpenRouter safeguards.
Fixes#88068.
Replaces #88071.