fix(tests): follow-ups from the timing-flakes review (#2150) #2155

Follow-up to #2150 (notes from the merge pass's review; none blocked CI).

• Direct runs can hit Bun's 5 s default timeout on a busy host (omo MEDIUM, server/codex.test.ts ~902-926): the ratio method makes the two bounded-time cases take ~1.3 s and ~0.9 s idle (vs 0.1/0.3 s before). bun test server/codex.test.ts without --timeout, as docs/development.md describes, failed 12/12 under a 12-copy load; CI and check fast pass --timeout 60000 and are unaffected. Give these cases an explicit timeout (as ci-browser.test.ts does) or shrink the inputs.
• The thread CPU clock helper in server/codex.test.ts ~21-33 is a copy of src/lib/markdown.test.ts ~248-259; share it.
• Without a thread clock (Windows) the fallback process.cpuUsage() counts all threads in ~15.6 ms steps, which could push the large-answer ratio near its bound.
• The ci-browser hang guard's error message drops stderr (scripts/ci-browser.test.ts ~90-91).