The retry loop added to survive a slow PowerShell teardown also made the
test unfalsifiable. The helper sleeps 5s, well inside the 30s deadline, so
a terminate that did nothing at all would let the sleep end on its own and
every existing assertion would still hold: the session reports exited, it
reports an exit code, and the interrupted flag echoes the request rather
than the outcome. Neutering KillProcessTree confirmed it, the test passed.
Assert instead that the helper's completion line never appears, which
distinguishes a killed session from a finished one without depending on
how long the loop took. Output is accumulated across every poll rather
than read off the last result, because each continuation returns only what
the session produced since the previous collection.