[ qingfeng/2020117 ] feat: support reject reason on DVM job rejection

Customer can now pass {"reason":"..."} when rejecting a result.
Reason is stored in the provider's job record and visible via job detail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/qingfeng/2020117/commit/4375369b3ae9b5841cdd7c754be3966b653a7c47