fix: reset isChildSession flag for root sessions

Address PR review feedback: The module-level isChildSession flag was only set
to true when a child session was detected, but never reset to false for root
sessions. This caused toast suppression to incorrectly persist after any child
session was created in the same plugin instance.

Now properly resets isChildSession=false and childSessionParentID=undefined
when a root session (no parentID) is created.