test(email): the deleted-alerts outbox test stays in one event loop (#5507)

Run 8942 failed this test alone (118 passed): it read the row back in a
second asyncio.run, and the app's pooled connection belonged to the
first, closed loop. The behaviour held ("The alerts this email carried
were deleted"); the settled row is now read from attempt's own result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_alert_email_outbox.py

@@ -156,14 +156,8 @@ def test_an_email_whose_alerts_were_deleted_fails_and_says_so(app, monkeypatch):
             async with s.begin():
                 await s.execute(delete(AlertEvent).where(AlertEvent.id == ev))
         done = await outbox.attempt(app, [row.id])
-        return done[row.id].state, row.id
+        return done[row.id]
 
-    state, rid = asyncio.run(_go())
-    assert state == "failed"
-
-    async def _reason():
-        from steward.models.outbox import EmailOutbox
-        async with app.db_sessionmaker() as s:
-            return (await s.get(EmailOutbox, rid)).last_error
-
-    assert asyncio.run(_reason()) == "The alerts this email carried were deleted"
+    # One event loop per test: the app's pool is bound to the loop it ran in.
+    row = asyncio.run(_go())
+    assert (row.state, row.last_error) == ("failed", "The alerts this email carried were deleted")