feat(alerts): a covering rule's hosts fold away behind its "N hosts" (#5483)

#5483 "Alerts list: a covering rule's hosts start collapsed behind a toggle;
no dashed rows", from the operator's review of the Alerts list. Each rule is
its own tbody; its hosts sit in another, hidden until "N hosts" is pressed.
A rule with a host that isn't quiet opens on its own and names the host where
"last fired" was, so a problem never hides behind the fold. Host rows lose the
dashed rules for a sunken band under the indent.

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

tests/core/test_alerts_page.py

@@ -292,7 +292,11 @@ async def test_a_rule_covering_hosts_lists_its_hosts_under_it(app):
         session["user_role"] = "operator"
         listing = await render_template(
             "alerts/list.html", active=[], rules=[parent], children={"p1": kids},
+            uneasy={"p1": ["node-a"]},
             rule_state={"p1": "firing", "k0": "firing", "k1": "inactive"}, operators=[])
+        quiet = await render_template(
+            "alerts/list.html", active=[], rules=[parent], children={"p1": kids},
+            uneasy={"p1": []}, rule_state={"p1": "inactive"}, operators=[])
         page = await render_template(
             "alerts/rules_form.html", rule=parent, operators=[], source_modules=["host_agent"],
             initial_source="host_agent", resources=[], metrics=["seconds_since_seen"],
@@ -305,6 +309,12 @@ async def test_a_rule_covering_hosts_lists_its_hosts_under_it(app):
     assert listing.index("Agent offline</a>") < listing.index('href="/alerts/history?rule=k0"') \
         < listing.index('href="/alerts/history?rule=k1"')
     assert 'class="row-link row-child"' in listing
+    # #5483: the hosts sit behind the rule's toggle, open while one isn't quiet.
+    assert 'aria-expanded="true" aria-controls="hosts-p1"' in listing and "on node-a" in listing
+    assert '<tbody id="hosts-p1" class="row-children">' in listing
+    quiet = " ".join(quiet.split())
+    assert 'aria-expanded="false" aria-controls="hosts-p1"' in quiet
+    assert '<tbody id="hosts-p1" class="row-children" hidden>' in quiet and "never fired" in quiet
     assert "Covers every host in <strong>swarm</strong>: 2 now (node-a, node-b)" in page
     assert 'name="covers" value="group"' in page and "fixed once the rule exists" in page
     assert 'data-cover-resource hidden' in page