Fix: Maintain cell focus when expanding hidden regions in notebooks

[Copilot]

When expanding hidden/folded regions in notebooks, the focused cell loses focus and focus moves to the notebook container. This breaks the expected interaction flow since the user is actively working with the cell.

Changes

foldingController.ts

• Added editor.focusElement(cell) after the notebook.unfold command expands cells, matching the existing pattern in notebook.fold

foldedCellHint.ts

• Added this._notebookEditor.focusElement(element) after the expand button handler expands the folded region

Both changes align with existing focus handling patterns: the folding icon click handler (line 142) and fold command (line 250) already maintain focus after collapsing.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Should cell input lose focus when hidden lines are expanded?</issue_title>
<issue_description>Testing #228393

• Focus the cell
• When you expand a hidden region, the focused cell loses focus and the whole notebook is focused.
• I am wondering if it would make sense to keep the focus on the cell since we interact with the cell?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Should cell input lose focus when hidden lines are expanded? #229486

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