Mark session as read when opened in Sessions window (#306603)

* Initial plan

* fix: mark session as read when opening it in Sessions window

When a session was marked as unread via the context menu, clicking on it
again did not change it back to read. This adds setRead(session, true) in
sessionsManagementService.openSession() and openChat(), following the same
pattern used in agentSessionsOpener.ts.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/04e61e19-5790-4bc5-8075-95457ae5adbd

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

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: bpasero <[email protected]>