Fixes#53527.
## Summary
- Suggest `untitled.<extension>` when saving an untitled editor buffer
with a selected non-Plain Text language.
- Preserve the existing title-based suggestion for existing files, Plain
Text buffers, and buffers without a language extension.
- Add a regression test for an untitled Rust buffer suggesting
`untitled.rs`.
## Testing
- `mise exec rust@1.95.0 -- cargo fmt --check -p editor`
- `mise exec rust@1.95.0 -- cargo test -p editor
test_suggested_filename_uses_language_extension_for_untitled_buffer
--lib`
## Suggested .rules additions
None.
Release Notes:
- Fixed Save As suggestions for untitled buffers with a selected
language.
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Co-authored-by: Kirill Bulatov <kirill@zed.dev>
