Announce resource tree selection state to screen readers (#4798482)

The Data Explorer left-nav resource tree did not expose selection state to
assistive technology. Screen readers announced tree items (e.g. "Documents,
1 of 2, level 3") without conveying whether the item was selected, failing
MAS 4.1.2 (Name, Role, Value).

Selection in the tree is a custom app-state concept (node.isSelected /
shouldShowAsSelected) and does not use Fluent v9's built-in selectionMode.
Wire the existing selection state to the aria-selected prop on TreeItem so
selectable nodes announce "selected"/"not selected". Pure grouping nodes
(no isSelected) omit the attribute, matching the ARIA single-select tree
pattern.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 956998dd-4849-4f7d-a027-b7190018e089

src/Explorer/Controls/TreeComponent/TreeNodeComponent.test.tsx

@@ -110,6 +110,42 @@ describe("TreeNodeComponent", () => {
     expect(component).toMatchSnapshot();
   });
 
+  it("sets aria-selected=true on a selected selectable node", () => {
+    const node = generateTestNode("root", {
+      isSelected: () => true,
+    });
+    const component = shallow(<TreeNodeComponent openItems={[]} node={node} treeNodeId={node.id} />);
+    expect(component.find(TreeItem).first().props()["aria-selected"]).toBe(true);
+  });
+
+  it("sets aria-selected=false on an unselected selectable node", () => {
+    const node = generateTestNode("root", {
+      isSelected: () => false,
+    });
+    const component = shallow(<TreeNodeComponent openItems={[]} node={node} treeNodeId={node.id} />);
+    expect(component.find(TreeItem).first().props()["aria-selected"]).toBe(false);
+  });
+
+  it("does not set aria-selected on a non-selectable (grouping) node", () => {
+    const node = generateTestNode("root");
+    delete node.isSelected;
+    const component = shallow(<TreeNodeComponent openItems={[]} node={node} treeNodeId={node.id} />);
+    expect(component.find(TreeItem).first().props()["aria-selected"]).toBeUndefined();
+  });
+
+  it("sets aria-selected=false on a selected parent when a descendant is selected", () => {
+    const node = generateTestNode("root", {
+      isSelected: () => true,
+      children: [
+        generateTestNode("child1", {
+          isSelected: () => true,
+        }),
+      ],
+    });
+    const component = shallow(<TreeNodeComponent openItems={[]} node={node} treeNodeId={node.id} />);
+    expect(component.find(TreeItem).first().props()["aria-selected"]).toBe(false);
+  });
+
   it("renders an icon if the node has one", () => {
     const node = generateTestNode("root", {
       iconSrc: "the-icon.svg",

src/Explorer/Controls/TreeComponent/TreeNodeComponent.tsx

@@ -172,6 +172,9 @@ export const TreeNodeComponent: React.FC<TreeNodeComponentProps> = ({
       itemType={isBranch ? "branch" : "leaf"}
       onOpenChange={onOpenChange}
       className={treeStyles.treeItem}
+      // Expose selection state to screen readers for selectable nodes so they announce "selected"/"not selected".
+      // Pure grouping nodes (no isSelected) omit the attribute, matching the ARIA single-select tree pattern.
+      aria-selected={node.isSelected ? shouldShowAsSelected : undefined}
     >
       <TreeItemLayout
         className={mergeClasses(

src/Explorer/Controls/TreeComponent/__snapshots__/TreeNodeComponent.test.tsx.snap

@@ -111,6 +111,7 @@ exports[`TreeNodeComponent fully renders a tree 1`] = `
   treeNodeId="root"
 >
   <TreeItem
+    aria-selected={false}
     className=""
     data-test="TreeNodeContainer:root"
     itemType="branch"
@@ -120,6 +121,7 @@ exports[`TreeNodeComponent fully renders a tree 1`] = `
     <div
       aria-expanded={false}
       aria-level={0}
+      aria-selected={false}
       className="fui-TreeItem r15xhw3a"
       data-fui-tree-item-value="root"
       data-test="TreeNodeContainer:root"
@@ -218,6 +220,7 @@ exports[`TreeNodeComponent fully renders a tree 1`] = `
               "current": <div
                 aria-expanded="false"
                 aria-level="0"
+                aria-selected="false"
                 class="fui-TreeItem r15xhw3a"
                 data-fui-tree-item-value="root"
                 data-test="TreeNodeContainer:root"
@@ -1595,6 +1598,7 @@ exports[`TreeNodeComponent renders an icon if the node has one 1`] = `
 
 exports[`TreeNodeComponent renders selected parent node as selected if no descendant nodes are selected 1`] = `
 <TreeItem
+  aria-selected={true}
   className=""
   data-test="TreeNodeContainer:root"
   itemType="branch"
@@ -1678,6 +1682,7 @@ exports[`TreeNodeComponent renders selected parent node as selected if no descen
 
 exports[`TreeNodeComponent renders selected parent node as unselected if any descendant node is selected 1`] = `
 <TreeItem
+  aria-selected={false}
   className=""
   data-test="TreeNodeContainer:root"
   itemType="branch"
@@ -1762,6 +1767,7 @@ exports[`TreeNodeComponent renders selected parent node as unselected if any des
 
 exports[`TreeNodeComponent renders single selected leaf node as selected 1`] = `
 <TreeItem
+  aria-selected={true}
   className=""
   data-test="TreeNodeContainer:root"
   itemType="leaf"