perf(tests): Improve local unit test performance

[janbuchar]

• pnpm test now runs two vitest projects — unit, then browser for the files that launch real browsers — each with its own worker cap derived from core count and RAM
• the is-ci check this replaces has been decorative since vitest renamed minThreads/maxThreads, so runs have been at one fork per core with browsers scattered across all of them for who knows how long
• The Playwright plugin matrix in plugins.test.ts is Chromium-only by default; CI and test:full run the full chromium,firefox,webkit via CRAWLEE_TEST_BROWSERS. Firefox and WebKit were ~94s of that file's ~109s (on my machine).
• Also gone: the esbuild block (oxc ignores it and warns on every run)

Wall time now floors out on the longest single file rather than on CPU, so extra workers only buy contention: 4 → 12 browser workers measured 127s → 111s for double the processes, and 10 → 16 unit workers measured 43.7s → 45.6s plus two contention failures. Splitting the Puppeteer and Playwright halves of browser-pool.test.ts (~86s, 88 sequential launches in one file) is the obvious next step if the browser group needs to get faster.