Pasting `line1\n\nline2` while the caret was inside a code block ran the
text through the Markdown parser, which split on the blank line and tore
the code block open, dropping the trailing content into a sibling
paragraph.
Detect the codeBlock parent on `handlePaste` and insert the clipboard
text verbatim instead. Code blocks have `code: true`, so newlines stay
literal — exactly what users expect when pasting code or logs.
Closes#1982