sessions: fix duplicate session in list when new session is opened while another is active (#314913)

sessions: fix duplicate session in list when new session opened while another is active

When a new CLI session was created and the user opened another session
before the new one fully graduated:

1. SessionAdded notification from the agent host arrived before
   _pendingSession was set, so _sessionCache already contained the
   committed session when the skeleton was added as _pendingSession.
   getSessions() then returned both, causing the duplicate row.

2. SessionsManagementService.onDidReplaceSession only fired
   _onDidChangeSessions when the active session was the 'from' session.
   If the user had navigated away, no refresh event fired and the
   SessionsList kept showing the duplicate indefinitely.

Fix:
- Clear _pendingSession before firing _onDidReplaceSession so any
  synchronous listener that calls getSessions() sees only the committed
  session (the finally block still clears it on the failure path).
- Always fire _onDidChangeSessions in onDidReplaceSession; only the
  active-session reassignment remains conditional.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>