mcs/scheduling: prevent scheduler update notifier from blocking (#10869)

[ti-chi-bot]

This is an automated cherry-pick of #10869

What problem does this PR solve?

Issue Number: Close #9776

What is changed and how does it work?

Make scheduler update notifications non-blocking so config updates only enqueue one pending signal when the scheduler updater is busy or the notifier channel is full.

Clear the scheduler update notifier registered by updateScheduler when the scheduling primary steps down, while preserving a newer notifier registered by a subsequent primary.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

Fix the issue that the scheduling service may stop working after switching primary while scheduler configs are updated.

Summary by CodeRabbit

• Bug Fixes

  • Notifications for scheduler updates are now non-blocking and coalesced to avoid hangs.
  • Notifier state is reliably cleared during shutdown or when replaced, improving cleanup.

• Tests

  • Added tests covering notifier lifecycle, replacement/clearing semantics, non-blocking behavior, and goroutine leak detection.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
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[APPROVALNOTIFIER] This PR is NOT APPROVED

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