@ file mentions: typing to filter doesn't match folder names, only filenames

Question

When using `@` mentions in the VS Code extension chat input:
- Typing `@` with no text shows folders correctly (e.g. `extensions/`, `extensions/OasisCalc/`)
- But typing to filter (e.g. `@exten`) only matches **filenames** containing that text, not **folder names**

This makes it impossible to quickly reference a subfolder by typing its name. You have to manually click through the folder tree instead.

**Expected:** `@exten` should also match folder names like `extensions/` in the autocomplete r