Fix: Remove edit instructions from readonly markdown cells in notebook diffs

[Copilot]

Empty markdown cells in readonly notebook diff viewers display "double-click or press enter to edit" despite being uneditable.

Changes

Modified BackLayerWebView._generateStyles() to conditionally set the empty cell placeholder based on readonly state:

'notebook-cell-markup-empty-content': this.notebookEditor.creationOptions.isReadOnly
    ? nls.localize('notebook.emptyMarkdownPlaceholderReadonly', "Empty markdown cell")
    : nls.localize('notebook.emptyMarkdownPlaceholder', "Empty markdown cell, double-click or press enter to edit."),

The CSS variable is consumed by the markdown renderer in extensions/markdown-language-features/notebook/index.ts which applies it as ::before pseudo-element content for .emptyMarkdownCell.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Change rendered text in readonly markdown code to not include or p;ress enter to edit</issue_title>
<issue_description>Open a diff virewer for notebooks with an empty md output.
The following is displayed

Given that the content is readonly, we should not include the text double-click or press enter to edit</issue_description>

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

• Fixes Change rendered text in readonly markdown code to not include or p;ress enter to edit #226909

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