Use utf8.RuneCountInString so multi-byte characters like é count as one
character. Add a unicode test case to cover this.
Signed-off-by: Émile Ré <emile@getprobo.com>
Add Node.TextLength() to walk the ProseMirror tree and sum actual user
text. Introduce ProseMirrorDocumentMaxTextLength validator that enforces
a 50k character limit on extracted text, paired with a 500k byte safety
cap on the raw JSON string. Extend e2e tests for content length.
Signed-off-by: Émile Ré <emile@getprobo.com>
