Handle when the bot labels are assigned manually

- When assigning `bot:merging` manually
- It should reject if there's already a merging PR in the process.
- It should process normally if there's no merging PR
- When assigning `bot:queued` manually
- It should become a merging PR when there's no merging PR and there's nothing in the queue

Start by tracing how the bot:merging and bot:queued labels are handled, then inspect the code that detects merging PRs and queued items. Verify each manual-label case: reject bot:merging when a merging PR exists, process it otherwise, and promote bot:queued when both the merging PR and queue are empty. Add or update tests for these three outcomes.

Written by the indexing model from the issue text.