This PR fixes#14040
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## Summary by CodeRabbit
* **New Features**
* Code blocks can be collapsed and expanded via a toolbar toggle
(visible when the document is editable).
* Collapsed code blocks show a limited preview (~8 lines) with a bottom
fade overlay and reduced padding.
* Toolbar button updates icon and tooltip to reflect collapsed/expanded
state.
* Collapse state is preserved on the block so its current
collapsed/expanded setting is retained.
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