Fix booking mock and ruff format drift exposed by PR #110 merge

test_spend_ceiling.py's budget-ceiling tests used a bare MagicMock
client, which broke once DealBookingFlow's booking path (from the
Meta integration work) started reading a real base_url and issuing
HTTP calls via _book_via_seller_api. Mock that call the same way the
rest of the suite does. Also applies a ruff-format-only fix to
helpers.py that was already failing `ruff format --check` on main.

Author: Sirajmx

src/ad_buyer/events/helpers.py

@@ -193,9 +193,7 @@ def emit_event_sync(
                 # writes the fallback record (best effort, logged if it fails).
                 future = asyncio.ensure_future(bus.publish(event))
                 if event_type in AUDIT_EVENT_TYPES:
-                    future.add_done_callback(
-                        lambda fut, ev=event: _audit_publish_done(fut, ev)
-                    )
+                    future.add_done_callback(lambda fut, ev=event: _audit_publish_done(fut, ev))
             else:
                 loop.run_until_complete(bus.publish(event))
         except RuntimeError:

tests/unit/test_spend_ceiling.py

@@ -13,7 +13,7 @@
 """
 
 import logging
-from unittest.mock import AsyncMock, MagicMock
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -250,7 +250,8 @@ def test_booking_at_budget_books(self):
         recs = [_make_recommendation("prod_a", "branding", 500000, 15.0)]  # cost 7500
         flow = self._flow_with_approved(7500, recs)
 
-        result = flow._execute_bookings()
+        with patch.object(flow, "_book_via_seller_api", return_value=("q", "d", "order_1")):
+            result = flow._execute_bookings()
 
         assert result["status"] == "success"
         assert result["booked"] == 1
@@ -264,7 +265,8 @@ def test_booking_under_budget_books(self):
         ]
         flow = self._flow_with_approved(100000, recs)
 
-        result = flow._execute_bookings()
+        with patch.object(flow, "_book_via_seller_api", return_value=("q", "d", "order_1")):
+            result = flow._execute_bookings()
 
         assert result["status"] == "success"
         assert result["booked"] == 2
@@ -275,7 +277,10 @@ def test_missing_budget_fails_open(self, caplog):
         recs = [_make_recommendation("prod_a", "branding", 500000, 15.0)]
         flow = self._flow_with_approved(None, recs)
 
-        with caplog.at_level(logging.WARNING):
+        with (
+            caplog.at_level(logging.WARNING),
+            patch.object(flow, "_book_via_seller_api", return_value=("q", "d", "order_1")),
+        ):
             result = flow._execute_bookings()
 
         assert result["status"] == "success"