ToolCallChunk

const leftChunks = [
  {
    name: "foo",
    args: '{"a":',
    index: 0
  }
];

const leftAIMessageChunk = new AIMessageChunk({
  content: "",
  tool_call_chunks: leftChunks
});

const rightChunks = [
  {
    name: undefined,
    args: '1}',
    index: 0
  }
];

const rightAIMessageChunk = new AIMessageChunk({
  content: "",
  tool_call_chunks: rightChunks
});

const result = leftAIMessageChunk.concat(rightAIMessageChunk);
// result.tool_call_chunks is equal to:
// [
//   {
//     name: "foo",
//     args: '{"a":1}'
//     index: 0
//   }
// ]