Remove worker idle/active callbacks that caused severe lock contention.
Instead, use sampling-based monitoring where worker states are queried
on-demand during scaling decisions. This eliminates the performance
bottleneck caused by workers acquiring locks 10+ times per second.
Changes:
- Remove callback parameters from Worker class
- Add properties to expose worker idle state directly
- Update WorkerPool to query worker states without callbacks
- Maintain scaling functionality with better performance
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