Frontend test: give the cold Vite SSR render in reasoning-source-render room on a loaded runner - #12903

Frontend unit tests (Windows) fails on PRs that do not touch the chat renderer (#12900 today, and #12875 before it) with:

not ok 7651 - source Markdown renders in an assistant root scope without a message part
  duration_ms: 30205.8217
  error: Command failed: node.exe --input-type=module --eval ...

Cause

The test, not the renderer. Each of the two tests in reasoning-source-render.test.ts (#10717) spawns a Node child that cold-starts a Vite SSR server and transforms the whole markdown-text.tsx graph with no prebundle, under a 30 s execFile timeout. That takes about 9 s per test on an idle Linux box. The Windows frontend job runs it alongside ~7,600 other tests, and on #12900 it took 30.2 s, so the child was killed by the timeout and the test failed with no assertion ever evaluated.

Fix

Both renders share one RENDER_TIMEOUT_MS = 120_000. The cap only exists to stop a hung child; what the test checks is the markup, and those assertions are unchanged.

Verification

• tests/reasoning-source-render.test.ts: 2 passed (about 9 s each locally).
• Lowering the cap to 3 s, standing in for a slow runner, fails both tests the way CI did; 120 s passes.
• Sabotage: making MarkdownTextSource render nothing fails both tests on this branch, so the change does not weaken what they guard.