In plan mode the active session model is the plan-role model, but
reassigning that role through the model hub only wrote settings and
never moved the live session onto the new model — planning continued on
the model plan mode was entered with until the next entry.
Subscribe InteractiveMode to onModelRolesChanged and, while plan mode is
active, re-resolve the plan role and switch onto it (deferring to the
next turn boundary when a turn is streaming). Extract the transition
decision into a pure resolvePlanModelTransition() helper with tests.
Fixes#5657
roboomp
97649df208