Enable concurrent vitest execution (#36998)

Enable
[`sequence.concurrent`](https://vitest.dev/config/sequence.html#sequence-concurrent)
to run all js tests in parallel. This will help catch potential
concurrency bugs in the future. The "Repository Branch Settings" test
was not concurrency-safe, it was refactored to remove shared mutable
state.

Co-Authored-By: Claude (claude-opus-4-6) <noreply@anthropic.com>

---------

Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>

vitest.config.ts

@@ -14,6 +14,9 @@ export default defineConfig({
     globals: true,
     watch: false,
     isolate: false,
+    sequence: {
+      concurrent: true,
+    },
   },
   plugins: [
     stringPlugin(),