fix(text): paste plain text instead of blocking paste in editable shapes (tldraw#8192)

When pasting tldraw clipboard data into an editable text shape (e.g. a
geo shape label or note), the paste was silently blocked. This PR fixes
it to extract and insert the plain text content from the clipboard
instead.

fixes tldraw#8187

### Change type

- [x] `bugfix`

### Test plan

1. Create two shapes with text (e.g. geo shapes with labels)
2. Select and copy one shape (Cmd+C)
3. Double-click the other shape to edit its text
4. Paste (Cmd+V) — the plain text from the copied shape should be
inserted

### Release notes

- Fix pasting into editable text shapes when clipboard contains tldraw
data


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: a small, localized change to paste handling for editable
text, with minimal impact outside the specific tldraw-clipboard paste
path.
> 
> **Overview**
> Fixes paste into editable text shapes when the clipboard contains
tldraw HTML data. Instead of silently blocking the paste,
`useEditablePlainText` now prevents the default and inserts the
clipboard’s `text/plain` content via
`document.execCommand('insertText')` to preserve undo support.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0fa808b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Ani Krishnan <98394024+anikrisn@users.noreply.github.com>

Author: steveruizok

packages/tldraw/src/lib/shapes/shared/useEditablePlainText.ts

@@ -152,7 +152,13 @@ export function useEditableTextCommon(shapeId: TLShapeId) {
 				const html = e.clipboardData.getData('text/html')
 				if (html) {
 					if (html.includes('<div data-tldraw')) {
+						// Paste the plain text data instead of the tldraw data
+						const plainText = e.clipboardData.getData('text/plain')
 						preventDefault(e)
+						if (plainText) {
+							// eslint-disable-next-line @typescript-eslint/no-deprecated -- best way to insert text with undo support
+							document.execCommand('insertText', false, plainText)
+						}
 					}
 				}
 			}