[功能请求] 输入框高亮 at 某人的部分

### QQ(TIM)版本/QQ(TIM) Version

9.0.0.14110

### 为什么你认为需要此功能

旧版（8.9.50）默认给 at 某人的部分加上了背景，新版本好像移除了这个特性

### 联系到哪些已经存在的bug或者功能

_No response_

### 具体功能需要/Specific features require

给输入框 at 某人的部分加上背景，好像可以通过 hook 相关的 `Span` 的与绘制有关的方法来实现

Start by locating the QQ/TIM 9.0.0.14110 input field and the relevant Span drawing methods mentioned in the issue. Verify how the older 8.9.50 behavior highlighted @ mentions, then determine whether the current input rendering can restore that background; done means the mentioned portion is visibly highlighted while typing.

Written by the indexing model from the issue text.