Fix misconfiguration of owner when creating sessions on behalf of another user

[achimnol]

If the admin creates a new session with custom owner-access-key, the session goes into an unusable state (disabled app launcher, etc.) in the perspective of the designated owner..

Initial code investigation reveals that there is a mismatch of owner_access_key (manually set) and UserScope (automatically set from the user who made the API request). We need to resolve UserScope from owner_access_key if owner_access_key is manually set before calling AgentRegistry.create_session() in the manager’s API handlers.