test(hub): stop the Chrome profile cleanup racing its own children

`terminate()` signals the parent only. Chrome's zygote, renderer and gpu
children outlive it by a moment and go on writing into the profile, so rmtree
walked a directory that gained a file between its readdir and its rmdir and
raised "Directory not empty". The probe exited non-zero, and every test in the
file errored at setup — intermittently, roughly one run in three, for a reason
nowhere near the chat code they were testing.

TemporaryDirectory(ignore_cleanup_errors=True) in all four probes that own a
profile: a few bytes left in a throwaway directory are harmless, failing the run
is not. `proc.wait()` after `kill()` was also missing — a killed process still
has to be reaped.

layout_probe.py never cleaned up at all (mkdtemp, no removal) and never waited
for Chrome; it leaked a profile into /tmp on every run.

Ten consecutive runs of test_chat_send.py are clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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