Fix: copy the text when the block's serialized content doesn't contain it - #84493

What?

Closes #84362

Why?

How?

Add a new helper function isTextPreservedOnBlockCopy() in use-clipboard-handler.js, which is added as a condition to isWholeSingleBlockCopy:

• It gets the text of serialize( block ) with stripHTML() and .trim(). This is the same text setClipboardBlocks() would put on the clipboard as text/plain.
• If the DOM selection has text but the serialized block has none, the block copy would lose it. The handler then returns early, and the text copy handler copies the selected text as usual.

Testing Instructions

1. Register a post meta field, for example in a mu-plugin:

   add_action( 'init', function () {
   	register_post_meta( 'post', 'test', array(
   		'label'        => 'Test field',
   		'single'       => true,
   		'type'         => 'string',
   		'show_in_rest' => true,
   		'default'      => 'Try to select and copy this text',
   	) );
   } );

2. Create a post and add a paragraph bound to it through the code editor
3. Click into the bound paragraph, select all of its text and copy it.
4. Paste into another paragraph or an external text editor. The text should be pasted. On trunk, nothing is pasted.

Screenshots or screencast

Use of AI Tools

Tool -> Claude Code
Model -> Opus 5.5
Use -> Research the issue and create a test case (all code manually verified and tested)