feat(concurrency): prevent background task races and leaks
Summary
Fixes race conditions and memory leaks in the background task system that could cause "all tasks complete" notifications to never fire, leaving parent sessions waiting indefinitely.
Why This Change?
When background tasks are tracked for completion notifications, the system maintains a pendingByParent map to know when all tasks for a parent session are done. Several edge cases caused "stale entries" to accumulate in this map:
1. Re-registering completed tasks added them back to pending tracking, but they'd never complete again
2. Changing a task's parent session left orphan entries in the old parent's tracking set
3. Concurrent task operations could cause double-acquisition of concurrency slots
These bugs meant the system would sometimes wait forever for tasks that were already done.
What Changed
- Concurrency management: Added proper acquire/release lifecycle with cleanup on process exit (SIGINT, SIGTERM)
- Parent session tracking: Fixed cleanup order. Now clears old parent's tracking before updating parent ID
- Stale entry prevention: Only tracks tasks that are actually running; actively cleans up completed tasks
- Renamed registerExternalTask → trackTask: Clearer name (the old name implied external API consumers, but it's internal)
Jeremy Gollehon
837176d947