Add single-line collapsed system test

Includes a workaround to force type-casting of Lookbook preview Boolean
arguments.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: myabc

previews/primer/open_project/border_box/collapsible_header_preview.rb

@@ -25,8 +25,8 @@ def playground(
             title: title,
             description: description,
             count: count,
-            collapsed: collapsed,
-            multi_line: multi_line
+            collapsed: coerce_bool(collapsed),
+            multi_line: coerce_bool(multi_line)
           })
         end
 
@@ -71,7 +71,7 @@ def with_description(multi_line: true)
               description: "This backlog is unique to this one-time meeting. You can drag items in and out to add or remove them from the meeting agenda.",
               count: nil,
               collapsed: false,
-              multi_line: multi_line
+              multi_line: coerce_bool(multi_line)
             }
           )
         end
@@ -87,10 +87,22 @@ def collapsed(multi_line: true)
               description: "This backlog is unique to this one-time meeting. You can drag items in and out to add or remove them from the meeting agenda.",
               count: nil,
               collapsed: true,
-              multi_line: multi_line
+              multi_line: coerce_bool(multi_line)
             }
           )
         end
+
+        private
+
+        # URL params are always strings; coerce to actual booleans before passing to the component.
+        def coerce_bool(value)
+          case value
+          when true, false then value
+          when "true" then true
+          when "false" then false
+          else false
+          end
+        end
       end
     end
   end

test/system/open_project/border_box/collapsible_header_test.rb

@@ -17,6 +17,15 @@ def test_renders_collapsed
     assert_no_text("This backlog is unique to this one-time meeting. You can drag items in and out to add or remove them from the meeting agenda.")
   end
 
+  def test_description_visible_when_collapsed_and_single_line
+    visit_preview(:collapsed, module_prefix: "border_box", multi_line: false)
+
+    # In single-line mode, the description stays on the same row as the title
+    # and remains visible even when collapsed (contrast with multi_line: true)
+    assert_selector(".CollapsibleHeader-description", visible: true)
+    assert_text("This backlog is unique to this one-time meeting.")
+  end
+
   def test_click_behaviour
     visit_preview(:default, module_prefix: "border_box")