test: suppress ty's invalid-assignment on the fake-session swap

coordinator is explicitly typed as LocalThingsCoordinator here, so ty
correctly sees _session's declared type (DtlsCoapSession | None) and
flags assigning a FakeObserveSession to it. The fixture's own
_connect_session replacement does the same swap without tripping ty,
but only because its self parameter is unannotated -- ty has nothing to
check the assignment against there. Deliberate here (this is the whole
point of the test: substitute a stand-in session), so silenced rather
than restructured; ty's --add-ignore confirmed the comment syntax
(ty: ignore[...], not the mypy-style type: ignore[...] used elsewhere
in this suite, which ty doesn't appear to honor for this rule).

tests/localthings/test_coordinator.py

@@ -772,7 +772,7 @@ async def test_attempt_observe_mode_discards_stale_commit_after_session_swap(
     other = FakeObserveSession()
 
     def _swap_session_mid_wait(subscribed, grace_period_s, success_fraction=None):
-        coordinator._session = other
+        coordinator._session = other  # ty: ignore[invalid-assignment]
         return True
 
     with patch.object(