assistant edit tool: Report when file is empty or doesn't exist (#27190)

Instead of just reporting a search match failure, we'll now indicate
whether the file is empty or exists to help the model recover better
from bad edits.


Release Notes:

- N/A
2e8c0ff244Agus Zubiaga committed on 3/20/2025, 1:42:10 PM· committed by GitHubparent4421bdd
1 file changedLine totals unavailable