fix(alerts): a rule's page doesn't wait for its metric preview (#5485); Send test up top (#5486)

#5485 "A rule's page waits up to 25 s for its metric preview: a cold 24 h
series read from plugin_metrics". Opening "Agent offline" took 25.6 s; a
cold day of one series measured 18.3 s over MCP, a warm one 63 ms. The
pickers still come with the page from plugin_metric_latest; the preview is
its own request (hx-trigger=load), showing "Reading the last day…" until it
lands. Picker swaps no longer read it either.

#5486 "Send test where it can be found": the button moves from inside "What
happens" to the page header beside History, its result under the verdict.

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

tests/integration/test_alert_rule_form.py

@@ -51,11 +51,14 @@ def test_the_pickers_and_the_preview_read_what_the_host_reports(app):
                         _csrf="test-token", login_at=time.time())
 
         resp = await client.get("/alerts/api/rule_target", query_string={
-            "source_module": "host_agent", "pick_host": host})
+            "source_module": "host_agent", "pick_host": host, "metric_name": "cpu_pct"})
         page = " ".join((await resp.get_data(as_text=True)).split())
         assert resp.status_code == 200
         assert f'<option value="{host}" selected>The whole host</option>' in page
         assert f'<optgroup label="Mounts"> <option value="{host}:/data" >/data</option>' in page
+        # #5485: the pickers come without the preview, which loads in its own request.
+        assert 'hx-get="/alerts/api/metric_preview" hx-trigger="load"' in page
+        assert "Reading the last day" in page and "Now <strong>" not in page
 
         resp = await client.get("/alerts/api/metric_preview", query_string={
             "source_module": "host_agent", "pick_host": host, "resource_name": f"{host}:/data",