Fold #125784's prompt matrix ("Channel hint.", "", None) into the existing
parametrized continuity test. "Channel hint." is the regression guard; ""
pins the plain pass-through (downstream consumers treat "" and None alike,
so it guards against an `or None` rewrite, not a behaviour difference); None
is the control that passes on main and head and keeps the parent-override
ephemeral invariant honest. Drop the negative-control assertions that could
not fail once the override check passed (a tautological inequality and stub
echoes), and the trailing blank line that tripped `git diff --check`.
Co-authored-by: funky-xamarin <30426178+Wenfengcheng@users.noreply.github.com>
58d146e961