## Why
Sibling tests can spawn children while an executable fixture is open for
writing, allowing inherited writable descriptors to cause `ETXTBSY` when
launching the fixture.
## What changed
Add a shared `copy_executable` helper for the daemon, doctor path safety, and
exec-server tests. On Linux, run `/bin/cp` in a separate process and wait for
it to exit before launching the fixture, then restore the source permissions
to account for restrictive umasks. Keep `std::fs::copy` on other platforms.
GitOrigin-RevId: 88f36826bbc6ab4e16e28ff02d5e7d6bd8dbf793
