Fix alarm wake predicate to avoid scheduler busy-spin

[viknash]

Summary

• fix �larm::sleep to evaluate the wake predicate (_wake_up_func()) instead of checking function-object existence
• this restores intended blocking behavior for idle worker threads
• reduces unnecessary CPU spinning when no work is available

Why this is high-impact, low-effort

• one-line change in the scheduler wait path
• directly affects every worker thread idle cycle
• preserves scheduler semantics while removing avoidable busy-loop behavior

[Copilot]

Pull request overview

This PR fixes task_scheduler::alarm::sleep so it evaluates the provided wake predicate (wake_up_function) rather than only checking whether the std::function object is non-empty, restoring proper blocking behavior for idle worker threads and avoiding unnecessary busy spinning.

Changes:

• Update alarm::sleep to loop on _wake_up_func() (predicate result) instead of _wake_up_func (function object presence).

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