## Summary

This fixes a minor bug I found #44981 

- Fix percent-encoded filenames appearing in agent mentions after
message submission.
- Decode file:// paths in MentionUri::parse using the existing
urlencoding crate (already used elsewhere in the codebase).
- Add tests for non-ASCII file URIs.

## Screenshots

<img width="409" height="116" alt="image"
src="https://github.com/user-attachments/assets/32ef033b-6232-47c5-80c7-d5247d5dae88"
/>