Problem:
The `nvim_get_chan_info()` terminal channel test used `shell-test INTERACT` to verify that `jobstop()` reports an unhandled `SIGHUP` as exit code `129`.
`INTERACT` reads from stdin with `fgets()`, so closing the PTY could race with `SIGHUP` delivery. If `fgets()` observed EOF first, `shell-test` exited normally with code `0`, causing intermittent failures on slower sanitizer builds.
Solution:
Add a `shell-test HOLD` mode that prints a readiness prompt and then waits without reading stdin. Use it for the `SIGHUP` assertion so PTY EOF cannot make the helper exit normally before the signal path is observed.
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