Fix TreeView selection when loader is replaced with content

When a loader (spinner/skeleton) node is selected and replaced with actual content,
the selection state (aria-selected) now properly transfers to the first loaded node.
This ensures keyboard navigation maintains the expected selection state.

Co-authored-by: francinelucca <[email protected]>

Author: Copilot

app/components/primer/alpha/tree_view/tree_view_sub_tree_node_element.ts

@@ -279,6 +279,15 @@ export class TreeViewSubTreeNodeElement extends HTMLElement {
           if (!content) return
 
           content.focus()
+          
+          // Set aria-selected on the newly focused node to maintain selection state
+          // when the loader is replaced with actual content
+          const treeView = this.closest('tree-view')
+          if (treeView instanceof TreeViewElement) {
+            const previousNode = treeView.querySelector('[aria-selected=true]')
+            previousNode?.setAttribute('aria-selected', 'false')
+            content.setAttribute('aria-selected', 'true')
+          }
         }
 
         this.#activeElementIsLoader = false