fix(cdk/tree): set index in context (#31709)

We already had the `index` inside `insertNode`, but we weren't passing it along to the context.

Fixes #31578.

Author: crisbeto

src/cdk/tree/tree.ts

@@ -603,6 +603,7 @@ export class CdkTree<T, K = T>
 
     // Node context that will be provided to created embedded view
     const context = new CdkTreeNodeOutletContext<T>(nodeData);
+    context.index = index;
 
     parentData ??= this._parents.get(key) ?? undefined;
     // If the tree is flat tree, then use the `getLevel` function in flat tree control