feat: use broker-authoritative qty in slots RPC holdings

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

tests/application/test_slots_handler.py

@@ -155,7 +155,8 @@ def test_warned_true_when_qty_diverges():
     resp = handler(_req(filter="holding"))
     s = resp.payload["slots"][0]
     # slot.qty=5 vs hub asset_quantity=3 → divergence
-    assert s["qty"] == 5.0
+    assert s["qty"] is None
+    assert s["slot_quantity"] == 5.0
     assert s["asset_quantity"] == 3.0
     assert s["warned"] is True