[FEA] Invalidate gRPC chunked-download sessions when deleting a job

Summary

DeleteResult should invalidate all active chunked-result download sessions associated with the deleted job.

Rationale

StartChunkedDownload snapshots completed result arrays into chunked_downloads. Deleting the job currently removes the tracker entry, pending data, and log file, but a client that already holds a download_id can continue retrieving the snapshotted result. This leaves deleted job data accessible and resident until the download session is finished or expires.

Affected areas

• cpp/src/grpc/server/grpc_service_impl.cpp
  • StartChunkedDownload
  • GetResultChunk
  • download-session lifecycle coordination
• cpp/src/grpc/server/grpc_job_management.cpp
  • delete_job cleanup path
• cpp/src/grpc/server/grpc_server_types.hpp
  • any required download-session metadata or lifecycle declarations
• gRPC integration tests covering DeleteResult and chunked downloads

Required changes

1. Associate each ChunkedDownloadState entry with its originating job_id.
2. During delete_job, remove every matching download session while holding chunked_downloads_mutex.
3. Coordinate StartChunkedDownload and deletion so that no session can be created after deletion completes, including concurrent requests.
4. Ensure subsequent GetResultChunk calls for an invalidated download_id return the existing unknown-download/not-found behavior.
5. Add integration coverage for deleting a job with an active chunked download and for the creation/deletion race.

Acceptance criteria

• After a successful DeleteResult, no active chunked-download session remains for that job.
• A previously issued download_id cannot return any further result data after deletion.
• Concurrent StartChunkedDownload and DeleteResult cannot leave a usable session for a deleted job.
• Existing chunked-download behavior for non-deleted jobs remains unchanged.

Backlinks

• Pull request: https://github.com/NVIDIA/cuopt/pull/1604
• Review discussion: https://github.com/NVIDIA/cuopt/pull/1604#discussion_r3634569391
• Requested by: @tmckayus