When a token has BoundChannelId set, the distributor now uses that channel
instead of the normal channel selection process. The bound channel is still
validated to ensure it is enabled and supports the requested model.

Changes:
- Add ContextKeyTokenBoundChannelId constant
- Set bound channel in context during token authentication
- Check and use bound channel in distributor before normal selection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>