fix: implement InsertValuesAtStem for HashedNode in Binary Trie (#554)

This fixes the "insertValuesAtStem not implemented for hashed node" error
that was blocking contract creation tests in Binary Trie mode.

The implementation follows the pattern established in InternalNode.GetValuesAtStem:
1. Generate the path for the node's position in the tree
2. Resolve the hashed node to get actual node data from the database
3. Deserialize the resolved data into a concrete node type
4. Call InsertValuesAtStem on the resolved node

This allows the Binary Trie to handle cases where parts of the tree are not
in memory but need to be modified during state transitions, maintaining the
lazy-loading design.

Author: CPerezz

trie/bintrie/hashed_node.go

@@ -46,8 +46,27 @@ func (h HashedNode) GetValuesAtStem(_ []byte, _ NodeResolverFn) ([][]byte, error
 	return nil, errors.New("attempted to get values from an unresolved node")
 }
 
-func (h HashedNode) InsertValuesAtStem(key []byte, values [][]byte, resolver NodeResolverFn, depth int) (BinaryNode, error) {
-	return nil, errors.New("insertValuesAtStem not implemented for hashed node")
+func (h HashedNode) InsertValuesAtStem(stem []byte, values [][]byte, resolver NodeResolverFn, depth int) (BinaryNode, error) {
+	// Step 1: Generate the path for this node's position in the tree
+	path, err := keyToPath(depth, stem)
+	if err != nil {
+		return nil, fmt.Errorf("InsertValuesAtStem path generation error: %w", err)
+	}
+
+	// Step 2: Resolve the hashed node to get the actual node data
+	data, err := resolver(path, common.Hash(h))
+	if err != nil {
+		return nil, fmt.Errorf("InsertValuesAtStem resolve error: %w", err)
+	}
+
+	// Step 3: Deserialize the resolved data into a concrete node
+	node, err := DeserializeNode(data, depth)
+	if err != nil {
+		return nil, fmt.Errorf("InsertValuesAtStem node deserialization error: %w", err)
+	}
+
+	// Step 4: Call InsertValuesAtStem on the resolved concrete node
+	return node.InsertValuesAtStem(stem, values, resolver, depth)
 }
 
 func (h HashedNode) toDot(parent string, path string) string {