Make (*WatchedFiles[T]).Clone propagate nil

[DanielRosenwasser]

Fixes #2248.

[Copilot]

Pull request overview

This PR fixes a panic that occurred when WatchEnabled is false but TypingsLocation is set. In this scenario, the typingsWatch field (and other watch fields) remain nil, but ATA can still attempt to clone them, causing a nil pointer dereference when acquiring the mutex.

• Added nil check to WatchedFiles[T].Clone() to propagate nil instead of panicking
• Added unit test to verify nil propagation behavior
• Added integration test to ensure ATA works correctly with WatchEnabled=false

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/project/watch.go	Added nil check in Clone method to prevent panic when called on nil receiver
internal/project/watch_test.go	Added unit test verifying Clone returns nil when called on nil WatchedFiles
internal/project/ata/ata_test.go	Added integration test ensuring ATA doesn't panic with WatchEnabled=false but TypingsLocation set