Accounts whose primary sending identity differs from their login (basic
auth registers accountId from the typed login; OAuth from the identity
email) were force-re-authed on switch because the guard derived the
connected id only from the primary-identity email. Collect every
server-confirmed identifier (JMAP Session.username + primary-identity
email) and only re-auth when the target matches none. Excludes the
constructor username so a real desync still trips. Adds
JMAPClient.getSessionUsername().