[release/9.0-staging] Fix Linux FileSystemWatcher silent stop after unpaired IN_MOVED_FROM

[Copilot]

Customer Impact

• Customer reported (FileSystemWatcher on Linux: event-processing thread exits silently after interleaved directory renames (unpaired IN_MOVED_FROM handling) - watcher stops receiving events, no Error raised. #133420)

The customer built a version-control system on .NET and relies on FileSystemWatcher to detect changes in users’ working trees.
On Linux, a merge that moves directories on both sides can silently stop the watcher, disabling change detection for every workspace open in the process with no error or indication that the watcher has failed.
Expected: the watcher continues reporting changes or raises an Error so the application can recover.

Regression

• Yes

Most likely Introduced by #9130 in .NET 9. It's not reproducible with .NET 11, because FileSystemWatcher got rewritten.

Testing

Verified using the customer-provided repro app from the issue.

No automated regression test was added because it would be flaky and since this component got rewritten in .NET 11, it's very unlikely for us to regress this code path in servicing branch.

Risk

Low. The change is a single-line, minimal-diff fix (return false → break)
confined to one foreach loop inside the Linux inotify event-processing state
machine. It only affects the narrow error/cleanup path for an unmatched
IN_MOVED_FROM event on a previously-watched subdirectory; the rest of the
control flow (watch removal, event dispatch, loop state reset) is unchanged.

Fixes #133420

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused control-flow fix preserves watcher processing after stale-watch cleanup.

Review effort: Lite
Findings: None

What changed in this PR

Fixes a Linux FileSystemWatcher issue where an unmatched directory rename could permanently stop event processing.

Changes:

• Replaces an unintended method return with a loop break.
• Allows stale-watch cleanup and subsequent event processing to continue.

File	Description
src/​libraries/​System.IO.FileSystem.Watcher/​src/​System/​IO/​FileSystemWatcher.Linux.cs	Corrects control flow during unmatched directory rename handling.

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[artl93]

@adamsitnik - please check build analysis.

[adamsitnik]

/ba-g the build analysis shows two failures and both of them are known and unrelated (I have no idea why BA is red despite this)