fix(audio-player): 修复切歌后播放速率恢复为1

[serious-snow]

• 在设置播放速率时同时更新 defaultPlaybackRate 属性

[gemini-code-assist]

Summary of Changes

Hello @serious-snow, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求旨在修复音频播放器中的一个关键缺陷，即用户自定义的播放速率在切换音轨后无法保持。通过确保在设置播放速率时同时更新 defaultPlaybackRate 属性，此更改保证了播放速率在整个播放会话中保持一致，从而提升了用户体验。

Highlights

• 修复音频播放速率重置问题: 解决了在切换歌曲后，音频播放速率会意外恢复到默认值1的问题。

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[gemini-code-assist]

Code Review

本次 PR 修复了切换歌曲后播放速率被重置为 1 的问题。通过在设置 playbackRate 的同时设置 defaultPlaybackRate，确保了在更换音频源后播放速率得以保持。这个修复是正确的且有效的。

此外，我发现 setRate 方法在 audioElement 未初始化时调用可能会导致运行时错误。我提供了一个建议来增加空值检查以提高代码的健壮性，避免潜在的程序崩溃。

[gemini-code-assist]

为了增强代码的健壮性，当前实现假定 this.audioElement 总是存在。然而，setRate 方法有可能在播放器完全初始化之前被调用，此时 this.audioElement 为 null，会导致运行时抛出 TypeError 错误。建议添加一个空值检查来防止程序崩溃。

请注意，简单地增加一个返回语句会静默地忽略在初始化前设置的播放速率。一个更完善的方案是：

1. 在 AudioElementPlayer 类中添加一个成员变量来缓存播放速率（例如 private rate: number = 1;）。
2. setRate 方法总是更新这个缓存的速率值，并且仅在 this.audioElement 存在时更新其属性。
3. 在 audioElement 初始化后（可能在 onGraphInitialized 方法中），使用缓存的速率值来设置其初始的 playbackRate 和 defaultPlaybackRate。

这种模式可以确保用户的速率设置不会丢失，并提高了代码的鲁棒性。

  public setRate(value: number): void {
    if (!this.audioElement) {
      return;
    }
    this.audioElement.playbackRate = value;
    this.audioElement.defaultPlaybackRate = value;
  }