fix(message): getContainer prop doesn't work

[kovsu]

close: #6937

1. In the useConfigInject.ts file, add props.getContainer to the return value of getPopupContainer so that we can correctly assign getPopupContainer in useMessage.tsx.

在 useConfigInject.ts 文件中将 props.getContainer 添加到 getPopupContainer 的返回中，这样我们才能正确的在 useMessage.tsx 中给 getPopupContainer 正确赋值

// useConfigInject.ts
const getPopupContainer = computed(
- () => props.getPopupContainer ?? configProvider.getPopupContainer?.value,
+ () => props.getContainer ?? props.getPopupContainer ?? configProvider.getPopupContainer?.value,
);

// useMessage.tsx
const { getPrefixCls, getPopupContainer } = useConfigInject('message', props);

2. Attempting to access the DOM elements of the page within the onBeforeMount lifecycle hook returns null. Therefore, this method should be called within the onMounted lifecycle hook.

在 onBeforeMount 生命周期函数中去获取页面的 DOM 元素，会得到 null，所以应该在 onMounted 生命周期中调用该方法

After:

[github-actions]

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