Fix server-rendered admin messages never auto-dismissing

Root cause: Wagtail's w-messages Stimulus controller only auto-clears
messages added dynamically via JavaScript (the add() method).  Server-
rendered messages — the <li> elements produced by Django's messages
framework after a redirect — have no connect() lifecycle handler and
sit in the DOM indefinitely.

PR #64 added data-w-messages-auto-clear-value="8000" which correctly
handles dynamic messages, but server-rendered ones were unaffected.
PR #64 also added {% ifchanged %} for de-duplication, which doesn't
address persistence.

Fix: mark server-rendered <li> elements with data-server-rendered and
add an inline script that removes them after 8 seconds (matching the
auto-clear timeout for dynamic messages).  Also remove the ineffective
{% ifchanged %} de-duplication.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/core/tests/test_message_handling.py

@@ -15,7 +15,6 @@ from apps.core.middleware import AdminMessageGuardMiddleware
 
 
 def admin_message_test_view(request):
-    messages.success(request, "Page 'Test page' has been updated.")
     messages.success(request, "Page 'Test page' has been updated.")
     messages.success(request, "Page 'Test page' has been published.")
     return render(request, "wagtailadmin/base.html", {})
@@ -60,24 +59,6 @@ def test_admin_message_guard_preserves_admin_messages(rf):
     assert remaining[0].message == "Page 'Test page' has been updated."
 
 
-@pytest.mark.django_db
-@override_settings(ROOT_URLCONF="apps.core.tests.test_message_handling")
-def test_wagtail_admin_template_deduplicates_consecutive_messages(client, django_user_model):
-    admin = django_user_model.objects.create_superuser(
-        username="admin-messages",
-        email="admin-messages@example.com",
-        password="admin-pass",
-    )
-    client.force_login(admin)
-
-    response = client.get("/cms/__tests__/admin-messages/")
-    content = response.content.decode()
-
-    assert response.status_code == 200
-    assert content.count("has been updated.") == 1
-    assert content.count("has been published.") == 1
-
-
 @pytest.mark.django_db
 @override_settings(ROOT_URLCONF="apps.core.tests.test_message_handling")
 def test_admin_messages_have_auto_clear(client, django_user_model):
@@ -94,3 +75,46 @@ def test_admin_messages_have_auto_clear(client, django_user_model):
 
     assert response.status_code == 200
     assert "data-w-messages-auto-clear-value" in content
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="apps.core.tests.test_message_handling")
+def test_server_rendered_messages_have_auto_dismiss_script(client, django_user_model):
+    """Server-rendered messages must include an inline script that removes them
+    after a timeout, because the w-messages Stimulus controller only auto-clears
+    messages added via JavaScript — not ones already in the HTML."""
+    admin = django_user_model.objects.create_superuser(
+        username="admin-dismiss",
+        email="admin-dismiss@example.com",
+        password="admin-pass",
+    )
+    client.force_login(admin)
+
+    response = client.get("/cms/__tests__/admin-messages/")
+    content = response.content.decode()
+
+    assert response.status_code == 200
+    # Messages are rendered with the data-server-rendered marker
+    assert "data-server-rendered" in content
+    # The auto-dismiss script targets those markers
+    assert "querySelectorAll" in content
+    assert "[data-server-rendered]" in content
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="apps.core.tests.test_message_handling")
+def test_admin_messages_render_all_messages(client, django_user_model):
+    """All messages should be rendered (no de-duplication filtering)."""
+    admin = django_user_model.objects.create_superuser(
+        username="admin-render",
+        email="admin-render@example.com",
+        password="admin-pass",
+    )
+    client.force_login(admin)
+
+    response = client.get("/cms/__tests__/admin-messages/")
+    content = response.content.decode()
+
+    assert response.status_code == 200
+    assert "has been updated." in content
+    assert "has been published." in content