fix(Clipboard): copying no longer includes unrelated ancestor nodes

[d3m1d0v]

Summary

When copying nested content (e.g. list items inside a blockquote or table cell), the clipboard incorrectly included outer container nodes. Now getSelectionContent finds the nearest complex: 'root' ancestor and slices from there, preserving only the meaningful structure.

Before / After

Copying list items inside a blockquote:

- > - item 1        ← blockquote leaked into clipboard
- > - item 2
+ - item 1           ← only the list is copied
+ - item 2

Changes

• Extracted getSelectionContent into selection-content.ts
• Fixed slicing logic to exclude unrelated outer containers
• Added unit tests for lists, blockquotes, tables, yfm-tables

[gravity-ui]

[gravity-ui]