Fix Archive All freezing for 200+ sessions by batching events

Add runBatch() to AgentSessionsModel to suppress onDidChangeSessions during
bulk operations, firing only once after the batch completes. This prevents
N tree rebuilds when archiving N sessions.

Also fix double event firing in setArchived (setRead was not passing
skipEvent), and add batch archive/unarchive methods through the sessions
service chain for the sessions window path.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/14d5d82b-5225-4add-88d3-971cc48ab2c7

Co-authored-by: bpasero <[email protected]>