acp_thread: Decode file:// mention paths so non-ASCII names render correctly (#44983)

## 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"
/>

Cargo.lock

@@ -37,6 +37,7 @@ dependencies = [
  "terminal",
  "ui",
  "url",
+ "urlencoding",
  "util",
  "uuid",
  "watch",