Behavior: Validates that the specified stage session is still active and valid using the provided authentication token and identifier. Returns 404 if the session is invalid or expired.

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

Side Effects: The session validity is checked but the session state may be updated as part of verification.

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 is invalid or has expired.