test(alerts): seed metrics the way ingest does, through record_metrics (#5473)

#5473 "Opening an alert rule takes up to 15 s". With the rule pickers now
reading plugin_metric_latest, nine integration tests failed on rule_fields:
"host_agent has no resource 'box-…'". They seeded a resource by adding a raw
PluginMetric row, which no production path does: host-agent ingest and
record_metric both go through record_metrics, which keeps the latest-value
table. The seeds now go through record_metrics too.

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

tests/integration/test_alert_dispatch.py

@@ -25,6 +25,7 @@ def app():
 
 async def _rule(app, tag, **extra):
     from steward.alerts import service as alert_svc
+    from steward.core.alerts import record_metrics
     from steward.models.metrics import PluginMetric
     from steward.models.users import User, UserRole
     res = f"box-{tag}"
@@ -33,8 +34,8 @@ async def _rule(app, tag, **extra):
             user = User(username=f"dp-{tag}", email=f"dp-{tag}@test", password_hash="x",
                         role=UserRole.admin)
             s.add(user)
-            s.add(PluginMetric(source_module="host_agent", resource_name=res,
-                               metric_name="cpu_pct", value=1.0))
+            await record_metrics(s, [PluginMetric(source_module="host_agent", resource_name=res,
+                                                  metric_name="cpu_pct", value=1.0)])
             await s.flush()
             fields = await alert_svc.rule_fields(
                 s, name=f"hot {tag}", source_module="host_agent", resource_name=res,