fix(comp:modal): draggable of modal with custom header slot doesn't work

[sallerli1]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added/updated or not needed
• Docs and demo have been added/updated or not needed

What is the current behavior?

具有 header 插槽的modal 组件，开启 draggable，拖拽不生效

What is the new behavior?

修复以上问题

Other information

1. 私有的 header 组件返回的vnode可能不是最终渲染的vnode，改用 getFirstValidVnode 获取第一个可用的vnode
2. header 组件的 header 插槽不再支持多节点组件

[idux-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.95%. Comparing base (4e5cdcc) to head (67882ab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1970   +/-   ##
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  Coverage   90.95%   90.95%           
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  Files         358      358           
  Lines       35691    35691           
  Branches     3889     3888    -1     
=======================================
  Hits        32463    32463           
  Misses       3228     3228           

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