test(chat): close the mid-turn send's remaining mutation gaps

The channel-honesty assertions only covered the split's trigger and one half
of each pair, so a menu item or glyph left on the insert copy slipped through.
Assert the opened item as well as the trigger, both icons, and a single
failure toast.

Add the connection-switch case for `steerAvailable`: the capability flags only
upgrade from a snapshot, so the per-connection reset is the only thing keeping
one session's channel off the next one.