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>