## Context

Zed runs external formatters as stdin/stdout filters: it writes the
current buffer to stdin and replaces the buffer with the formatter's
stdout. Commands such as `cargo fmt` instead rewrite files on disk and
exit successfully without producing stdout, causing Zed to interpret the
empty output as the formatted contents and clear a non-empty buffer.

The fix treats empty stdout from a successful external formatter as no
output when the original buffer is non-empty. Zed leaves the buffer
unchanged and displays a notification explaining that the formatter did
not return formatted contents. The formatter documentation now clarifies
the stdin/stdout contract and recommends using the Rust language server
or invoking `rustfmt` directly.

Closes #56344

Behavior before the fix :

[Screencast from 2026-07-19
02-58-29.webm](https://github.com/user-attachments/assets/61fc5bf9-4420-43f1-94a3-394bbbc4f2a0)


Behavior after the fix :

[Screencast from 2026-07-19
02-55-35.webm](https://github.com/user-attachments/assets/21062065-6b95-4cd4-8200-506f5681d98e)


## How to Review

**crates/project/src/lsp_store.rs**  
Start with `format_via_external_command`, which now checks whether a
successful formatter returned empty stdout while the input buffer was
non-empty. In that case, it returns `None` before constructing a diff,
preventing the buffer contents from being replaced with an empty string.
Then review the external formatter branch in `apply_formatter`: when it
receives `None`, it skips extending the formatting transaction, logs the
condition, and emits an `LspStoreEvent::Notification` explaining why the
buffer was left unchanged.

**crates/editor/src/editor_tests.rs**  
Adds a GPUI regression test that configures an external command to
consume stdin and return no stdout. It uses platform-specific commands
for Windows and Unix, invokes manual formatting on a non-empty Rust
buffer, and verifies that the buffer remains unchanged, the operation is
not recorded as a formatter failure, and a notification is emitted.

**docs/src/reference/all-settings.md**  
Extends the external formatter documentation to state that formatters
must return the formatted buffer through stdout. It calls out
file-rewriting tools such as `cargo fmt` as incompatible and recommends
using the Rust language server or `rustfmt --emit stdout`.

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed external formatters that produce no output clearing non-empty
buffers