Closes #60041.

## Summary

When Project Search results contain both a saved file and an untitled
buffer, running `workspace: reload` did nothing (quitting hit the same
path). The log showed:

```
ERROR [.../workspace.rs] buffer doesn't have a file
```

## Root cause

The Project Search results view is an `Editor` over a multi-buffer, and
its `Item::save` delegates to `Editor::save`. For a multi-buffer,
`Editor::save` collects every dirty buffer and calls
`Project::save_buffers`. An untitled excerpt has no file, so
`BufferStore::save_buffer` returns `Err("buffer doesn't have a file")`.
That error propagates through `Pane::save_item` → `save_all_internal` →
`prepare_to_close`, which then reports the workspace as not ready to
close, so `reload`/quit silently aborts.

A singleton untitled buffer avoids this because `Editor::can_save`
returns `false` for it (the workspace routes it to `save_as`); for a
multi-buffer, `can_save` is always `true`.

## Fix

Exclude file-less (untitled) buffers when collecting the buffers to save
for a multi-buffer. Untitled buffers can only be written via `save_as`,
so a bulk multi-buffer save now persists the file-backed excerpts and
leaves untitled ones untouched. This also addresses the same latent
issue in other multi-buffer views (diagnostics, find-all-references)
that can excerpt an untitled buffer.

## Testing

- Added `test_save_multi_buffer_with_untitled_buffer_skips_untitled` in
`crates/editor/src/items.rs`: it builds a multi-buffer over a
file-backed buffer and an untitled buffer (both dirty), then asserts
`save` succeeds, the file-backed buffer is persisted, and the untitled
buffer stays dirty. Fails before this change (`buffer doesn't have a
file`), passes after.
- `cargo test -p editor --lib save` (13 tests) and `cargo test -p search
--lib project_search` (22 tests) pass.
- `./script/clippy -p editor` is clean.

Manual repro (before): new untitled buffer with text → Project Search
for a term matching both it and a saved file → `workspace: reload` →
no-op with `buffer doesn't have a file` in the log. After: reload
proceeds.

Release Notes:

- Fixed `workspace: reload` and quitting silently doing nothing when
Project Search results included an unsaved untitled buffer.