[multistage] query cancel for multi-stage engine

https://github.com/apache/pinot/pull/9171 added support for query cancelation

This is a bit more complex in multi-stage engine as there are multiple stage in various distributed machines.

This issue tracks the effort to enable query cancellation on multi-stage engine
- [ ] managed running tasks on multistage `QueryServer` which is the GRPC entrypoint
- [ ] periodic clean up task for finished jobs (such as mailbox to channel hash map)
- [ ] allow multi-stage engine to issue cancellation GRPC proto service to `QueryServer`
- [ ] enable `QueryDispatcher` endpoint to do query cancellation

Start by reading PR 9171 and the multi-stage engine's QueryServer and QueryDispatcher entry points. Trace how running tasks, mailboxes, and channels are managed across distributed machines, then review the gRPC cancellation path. Done means all four checklist items are implemented and multi-stage queries can be cancelled and cleaned up reliably.

Written by the indexing model from the issue text.