test: relaunch a disconnected cached browser in tests

[thomasbachem]

The Windows screenshot flakes in #2137 are TargetCloseErrors (Target closed), which is what you get from reusing a browser that already died.

withBrowser (tests/utils.ts) caches one browser per launchOptions key, but the cache-hit path only checks that an entry exists, not that it's still connected – and nothing evicts a dead one. So a single browser death mid-run (a pipe drop, an OOM, a teardown race) leaves every later same-key test reusing the dead handle, turning one failure into a run of them.

#2205's concurrency limit lowers how often a browser dies; a liveness check on cache hit bounds the blast radius when one still does – a disconnected browser is dropped and relaunched, so a death fails only its own test. Inert on a healthy run, since connected only goes false once the connection is actually gone.

The test closes a cached browser and asserts the next withBrowser call gets a fresh, connected one – it fails on main and passes with the check.

[OrKoN]

Thanks!