refactor floating_domain to structured object-only payload

tests/test_output_formatter.py

@@ -41,11 +41,18 @@ async def test_stream_formatter_floating_domain_first() -> None:
     formatter = StreamFormatter(request_id="req-2")
     frames = await _collect(
         formatter,
-        _stream(("floating_domain", "notes"), ("token", "Summary")),
+        _stream(
+            (
+                "floating_domain",
+                {"type": "node", "id": "n-1", "section": None},
+            ),
+            ("token", "Summary"),
+        ),
     )
 
     assert isinstance(frames[0], WsFloatingDomain)
-    assert frames[0].domain == "notes"
+    assert frames[0].domain.type == "node"
+    assert frames[0].domain.id == "n-1"
     assert isinstance(frames[1], WsStreamStart)
     assert isinstance(frames[2], WsStreamText)
     assert frames[2].chunk == "Summary"