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>

steward/templates/alerts/_rule_target.html

@@ -3,7 +3,8 @@
    so on), so a host's parts never crowd the list of hosts. Any other source:
    one Resource picker. Then the metrics that resource reports, and a preview
    of the chosen one. Picking a host or part swaps this block alone
-   (/alerts/api/rule_target); picking a metric swaps the preview. #}
+   (/alerts/api/rule_target); picking a metric swaps the preview, which
+   is read in its own request after the block is drawn (#5485). #}
 {% set one = current_covers|default("resource") == "resource" %}
 <div data-cover-resource {% if not one %}hidden{% endif %}>
   {% if host_source %}
@@ -68,5 +69,10 @@
   {% if metric and metric_about(metric) %}
   <p class="field-hint" data-metric-about>{{ metric_about(metric) }}</p>
   {% else %}<p class="field-hint" data-metric-about hidden></p>{% endif %}
-  <div id="metric-preview" aria-live="polite">{% include "alerts/_metric_preview.html" %}</div>
+  {# The preview comes in its own request once the page is up (#5485). #}
+  <div id="metric-preview" aria-live="polite"
+       {%- if metric and resource %} hx-get="/alerts/api/metric_preview" hx-trigger="load"
+       hx-include="closest form" hx-params="source_module,resource_name,pick_host,metric_name,covers"{% endif %}>
+    {%- include "alerts/_metric_preview.html" -%}
+  </div>
 </div>