Allow to fetch cargo diagnostics separately (#29706)

Adjusts the way `cargo` and `rust-analyzer` diagnostics are fetched into
Zed.

Nothing is changed for defaults: in this mode, Zed does nothing but
reports file updates, which trigger rust-analyzers'
mechanisms:

* generating internal diagnostics, which it is able to produce on the
fly, without blocking cargo lock.
Unfortunately, there are not that many diagnostics in r-a, and some of
them have false-positives compared to rustc ones

* running `cargo check --workspace --all-targets` on each file save,
taking the cargo lock
For large projects like Zed, this might take a while, reducing the
ability to choose how to work with the project: e.g. it's impossible to
save multiple times without long diagnostics refreshes (may happen
automatically on e.g. focus loss), save the project and run it instantly
without waiting for cargo check to finish, etc.

In addition, it's relatively tricky to reconfigure r-a to run a
different command, with different arguments and maybe different env
vars: that would require a language server restart (and a large project
reindex) and fiddling with multiple JSON fields.

The new mode aims to separate out cargo diagnostics into its own loop so
that all Zed diagnostics features are supported still.


For that, an extra mode was introduced:

```jsonc
"rust": {
  // When enabled, Zed runs `cargo check --message-format=json`-based commands and
  // collect cargo diagnostics instead of rust-analyzer.
  "fetch_cargo_diagnostics": false,
  // A command override for fetching the cargo diagnostics.
  // First argument is the command, followed by the arguments.
  "diagnostics_fetch_command": [
    "cargo",
    "check",
    "--quiet",
    "--workspace",
    "--message-format=json",
    "--all-targets",
    "--keep-going"
  ],
  // Extra environment variables to pass to the diagnostics fetch command.
  "env": {}
}
```

which calls to cargo, parses its output and mixes in with the existing
diagnostics:




https://github.com/user-attachments/assets/e986f955-b452-4995-8aac-3049683dd22c




Release Notes:

- Added a way to get diagnostics from cargo and rust-analyzer without
mutually locking each other
- Added `ctrl-r` binding to refresh diagnostics in the project
diagnostics editor context

crates/multi_buffer/src/multi_buffer.rs

@@ -1686,7 +1686,10 @@ impl MultiBuffer {
         let mut counts: Vec<usize> = Vec::new();
         for range in expanded_ranges {
             if let Some(last_range) = merged_ranges.last_mut() {
-                debug_assert!(last_range.context.start <= range.context.start);
+                debug_assert!(
+                    last_range.context.start <= range.context.start,
+                    "Last range: {last_range:?} Range: {range:?}"
+                );
                 if last_range.context.end >= range.context.start {
                     last_range.context.end = range.context.end.max(last_range.context.end);
                     *counts.last_mut().unwrap() += 1;