mvcc: fix watch compaction race causing double notifications

[Aman-Cool]

This PR documents and fixes a correctness issue in the watch compaction path.

Issue

When the number of unsynced watchers was below the sync limit, choose() returned the original watcher group instead of a copy. During compaction, this caused the watcher map to be modified while it was being iterated, which could lead to:

• etcd panicking with removing missing watcher!
• Watchers silently missing compaction notifications and hanging indefinitely

This can occur during normal operation with auto-compaction enabled.

Fix

• choose() now always returns a copy of the watcher group
• chooseAll() no longer deletes watchers while iterating
• Compacted watchers are cleaned up safely in syncWatchers()

Result

The fix eliminates panics and silent watch failures while preserving existing behavior.
All relevant MVCC tests pass, including compaction-related watch tests.

[k8s-ci-robot]

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[k8s-ci-robot]

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[Aman-Cool]

@serathius @ahrtr , This PR fixes a watch compaction bug where chooseAll() could mutate the unsynced watcher map during iteration when the watcher count is small, leading to panics or silent watch hangs.
The fix always works on a copy and defers cleanup to syncWatchers().