perf(oxlint): render JSON report into a single buffer (#25295)

AI Disclosure: Generated with Claude Code, Opus 5. Reviewed and tested by me.

## What

`format_json` rendered each diagnostic into its own `String`, collected them into a `Vec<String>`, then `join`ed them. That allocates once per diagnostic and copies the entire report a second time to build the joined result.

This renders straight into one output buffer instead.

## Why

On the vscode corpus with `-D all` — 1.37M diagnostics, ~500 MB of JSON output:

| | before | after |
| --- | --- | --- |
| `--format=json` wall clock | 13.3 s | 12.6 s |
| peak RSS | 4668 MB | 3576 MB |

Measured as best-of-3 with the two binaries interleaved, output redirected to `/dev/null`.

## Correctness

Output is byte-identical before and after. Verified on `apps/oxlint/fixtures/cli/output_formatter_diagnostic` and on a 365k-diagnostic subset of vscode (170 MB of JSON compared byte-for-byte). Note that oxlint's diagnostic ordering is nondeterministic run-to-run under parallelism, so the comparison was run with `--threads=1`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

apps/oxlint/src/output_formatter/json.rs

@@ -139,16 +139,15 @@ impl JsonReporter {
 /// <https://github.com/fregante/eslint-formatters/tree/ae1fd9748596447d1fd09625c33d9e7ba9a3d06d/packages/eslint-formatter-json>
 fn format_json(diagnostics: &mut Vec<Error>) -> String {
     let handler = JSONReportHandler::new();
-    let messages = diagnostics
-        .drain(..)
-        .map(|error| {
-            let mut output = String::new();
-            handler.render_report(&mut output, error.as_ref()).unwrap();
-            output
-        })
-        .collect::<Vec<_>>()
-        .join(",\n");
-    format!("[{messages}]")
+    let mut output = String::from("[");
+    for (index, error) in diagnostics.drain(..).enumerate() {
+        if index > 0 {
+            output.push_str(",\n");
+        }
+        handler.render_report(&mut output, error.as_ref()).unwrap();
+    }
+    output.push(']');
+    output
 }
 
 #[cfg(test)]