Fix DiffPipeline.ApplyPatch: handle differs that do internal atomic replacement

[JusterZhu]

Summary

Fix DiffPipeline.ApplyPatch to correctly handle IBinaryDiffer implementations that perform atomic file replacement internally (e.g., BinaryHandler).

Bug

BinaryHandler.DirtyAsync already does: delete old file, copy temp to old, delete temp. After it returns, tempPath no longer exists. DiffPipeline.ApplyPatch then:

1. Deletes old file (which was just restored)
2. Tries File.Move(temp, old) — temp doesn't exist — FileNotFoundException

Fix

ApplyPatch now checks if tempPath still exists after DirtyAsync:

• Exists: differ left tempPath intact — do atomic replacement
• Does not exist: differ already handled it — old file is already correct