* Split `shutdown` into two separate methods for better control:
- `shutdown` - signals threads to stop accepting new work
- `wait_for_termination` - waits for threads to finish (with optional timeout)
* Add tracking of busy threads via `@busy_threads` Set
* Make idle_time parameter optional with 30-second default
* Improve thread spawning logic:
- Spawn initial thread immediately when work is posted
- Spawn additional threads when all threads are busy and work is queued
* Fix race condition in work distribution
* Add busy thread count to stats output
* Add test coverage for zero min_threads configuration
This commit makes the ThreadPool more reliable, easier to use, and adds
better visibility into its internal state.
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Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>
c315e26485