Behavior: Initiates a new stage session using the provided authentication token and identifier. The token is normally supplied by the calling system as the unique identifier of the experience. Returns 404 if the session cannot be started.

Authorization: Requires an authenticated user (class-level [Authorize]).

Side Effects: A new stage session is created for the authenticated user.

Error Scenarios: Returns 400 if the input model is null or invalid. Returns 401 if the user is not authenticated. Returns 404 if the session could not be started.