Visual Editor incorrectly escapes '@' in Pandoc citations on drag-and-drop

[ronblum]

Bug description

The following was reported by @crocodilusniloticus in RStudio repo rstudio/rstudio rstudio/rstudio#16537

When dragging a plain-text Pandoc citation (e.g., [@smith2023]) from an external application like Zotero into the RStudio Visual Editor, the editor incorrectly escapes the at-sign, resulting in \[@smith2023].

Steps to Reproduce:
1. Configure Zotero with Better BibTeX to drag citations as plain-text Pandoc citations.
2. Drag a reference from Zotero into a plain text editor (like the RStudio Source Editor) to confirm it is correctly formatted as [@key].
3. Drag the same reference into the RStudio Visual Editor.

**Expected Behavior:**
The editor should insert the plain text as-is: `[@key]`

**Actual Behavior:**
The editor inserts escaped text: `\@key`, which breaks the Pandoc citation functionality."

Steps to reproduce

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Actual behavior

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Expected behavior

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Your environment

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