send_to_session let an authenticated caller reach a null-owner session.
The owner gate was `if owner and sess.owner and sess.owner != owner`, so a
target whose owner is None (legacy rows, or a session created while auth
was off) skipped the check and was read/written by any authenticated user.
list_sessions (get_sessions_for_user) and manage_session already exclude
null-owner sessions from an authenticated caller via an exact owner match,
so this path was the lone inconsistency — the same class of gap the
calendar owner=None fix closed.
Require an exact owner match: `if owner and sess.owner != owner`. Auth-off
(no owner) is unchanged, an exact-owner match still passes, and both
another user's session and a null-owner session are now not-found. Adds a
regression test that an authenticated caller cannot read the transcript of
or write into a null-owner session while single-user access still works.
