Add response appearing effect

### 需求动机

请参考腾讯元宝的回复效果。文字的 opacity 逐渐从 0 变为 1。这种 appearing 的效果给人一种逐渐思考的感觉，非常适合大模型回复这种场景

### 提议的 API 是什么样的？

可以在 BubbleProps 的 typing 里面增加一个 appearing

Start by locating the BubbleProps typing and the response-rendering entry point. Confirm how the proposed appearing prop should control the requested opacity transition, then use the project's existing checks to verify that the prop is exposed and responses transition from opacity 0 to 1.

Written by the indexing model from the issue text.