`hermes desktop` / `hermes update` recover from a corrupt Electron download by
purging the cached zip + re-downloading and retrying the pack, and then by
falling back to a public mirror. That recovery is only meaningful when the
packaged executable is MISSING — the signature of a partial/corrupt unpack.
A LATE failure such as macOS code signing (#40187) leaves
`Hermes.app/Contents/MacOS/Hermes` (or the platform equivalent) in place.
Re-downloading Electron can't repair a signing failure, so the purge +
slow mirror retry just grind through another identical failure before the
build finally errors out.
Gate both recovery blocks on `_desktop_packaged_executable(desktop_dir) is None`
so a build that already produced the executable fails fast instead of
triggering the destructive download recovery. The corrupt-download path
(executable missing) is unchanged.
Salvage of #42782, re-applied onto current main (the surrounding recovery was
refactored to `_electron_dist_ok` / `_redownload_electron_dist` since the PR
was opened). Adds a regression test asserting no purge / mirror retry runs when
the executable exists, and updates the existing retry/mirror tests to model the
corrupt-download case (executable absent) the recovery is actually for.
Related to #40187 (the residual cache-purge sub-issue; the signing failure
itself is fixed by #52591).
ailthrim
25ec01f79f