forkBranch ended by opening the branch as a session-tile and leaving the
primary selection on the parent (#69750). In the default layout there is
no visible tile pane, so branching only added a sidebar row with no
feedback in the main area — and openSessionTile no-ops when the target
is already the selected session, the common case of branching the chat
you're viewing.
Load the branch as the primary session via resumeSession instead, which
reuses the runtime already warm-cached by forkBranch's
ensureSessionState/updateSessionState calls, so it doesn't cost an extra
resume RPC.
Fixes#93444
chelsealong
a4092cd2d2