Make formatter output quiet on success (#29467)

## Why

`just fmt` is quite noisy even on successful runs.

## What

Only print output when a formatter fails.

- Buffer output from each formatter and print only a failed command and
its diagnostics.
- Prefix the `justfile` driver invocations with `@` so Just does not
echo the command itself.
- Retain rustfmt stderr on failure and cover silent-success and
failure-reporting behavior.

## Validation

- Confirmed `just fmt` and `just fmt-check` both exit successfully with
empty stdout and stderr.

justfile

@@ -38,11 +38,11 @@ app-server-test-client *args:
 
 # Format the justfile, Rust, Bazel/Starlark, Python SDK code, and Python scripts.
 fmt:
-    {{ python }} ../scripts/format.py
+    @{{ python }} ../scripts/format.py
 
 # Check formatting without modifying files.
 fmt-check:
-    {{ python }} ../scripts/format.py --check
+    @{{ python }} ../scripts/format.py --check
 
 fix *args:
     cargo clippy --fix --tests --allow-dirty {args}