fix(runner): release fs watcher lock on removal errors

[linkdata]

Summary

• release cleanupFSWatchers RLock with defer on all exit paths
• add a small removal hook so the error path can be exercised directly
• assert the mutex is released after a watcher removal failure

Tests

• go test ./pkg/runner ./...

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• pkg/runner/runner.go
• pkg/runner/runner_test.go

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[linkdata]

Additional context from comment files

These notes were split from the local markdown comment files and attached here because they describe this PR's change.

FS Watcher Mutex Leak

• Bug: In cleanupFSWatchers(), an early return statement inside the RLock scope at lines 1003–1005 exited the function without calling RUnlock(), leaking the read lock.
• Impact: Any subsequent attempt to acquire the write lock on fsWatcherMutex (e.g., in configureFSWatchers) would block indefinitely because the read lock was never released.
• Fix: Added defer edm.fsWatcherMutex.RUnlock() immediately after RLock() and removed the explicit RUnlock() call that was only reachable on the success path.
• Reasoning: Using defer ensures the lock is always released, even on error return paths, preventing mutex deadlocks.
• Tests: Added TestCleanupFSWatchersReleasesLockOnError which calls cleanupFSWatchers() and then verifies fsWatcherMutex.TryLock() succeeds, confirming the read lock was released.