fix: find and add missing variant in synthetic messages

[cau1k]

If a request includes a variant field, finds and adds the correct variant to the agent message. With every Claude Code plan model, I would always get plaintext reasoning (<thinking> ... </thinking>) blocks in message outputs and errors similar to:

messages.1.content.0: When thinking is disabled, an `assistant` message in the final position cannot contain `thinking`. To use thinking blocks, enable `thinking` in your request.

Looked through my sessions with DCP enabled and eventually found the variant field was being set to null in synthetic messages, causing OC to throw non-retryable errors.

edit: example of the issue

[Tarquinen]

moved to #224, thanks for the fix!