[5.1.x] Fixed #35606, Refs #34045 -- Fixed rendering of ModelAdmin.action_checkbox for models with a __html__ method.

Thank you Claude Paroz for the report.

Regression in 85366fbca7.

Backport of 182f262b15 from main.

tests/admin_changelist/tests.py

@@ -364,6 +364,33 @@ class ChangeListTests(TestCase):
             table_output,
         )
 
+    def test_action_checkbox_for_model_with_dunder_html(self):
+        grandchild = GrandChild.objects.create(name="name")
+        request = self._mocked_authenticated_request("/grandchild/", self.superuser)
+        m = GrandChildAdmin(GrandChild, custom_site)
+        cl = m.get_changelist_instance(request)
+        cl.formset = None
+        template = Template(
+            "{% load admin_list %}{% spaceless %}{% result_list cl %}{% endspaceless %}"
+        )
+        context = Context({"cl": cl, "opts": GrandChild._meta})
+        table_output = template.render(context)
+        link = reverse(
+            "admin:admin_changelist_grandchild_change", args=(grandchild.id,)
+        )
+        row_html = build_tbody_html(
+            grandchild,
+            link,
+            "name",
+            '<td class="field-parent__name">-</td>'
+            '<td class="field-parent__parent__name">-</td>',
+        )
+        self.assertNotEqual(
+            table_output.find(row_html),
+            -1,
+            "Failed to find expected row element: %s" % table_output,
+        )
+
     def test_result_list_editable_html(self):
         """
         Regression tests for #11791: Inclusion tag result_list generates a