Function append_text 

pub fn append_text(content: &mut Value, text: &str) -> bool

Append text to content as a trailing piece of text, in either shape, and say whether it could be.

A string gains text after a blank line. An array gains one more {"type": "text", "text": …} part at the end, beside whatever parts it already has — the walks above can only rewrite text that is already there, so adding a piece is its own operation rather than a use of them. Any other shape — null, absent, a number — is left untouched and reports false, which is the caller’s signal that this message cannot carry the text and something else must.

The blank line matters: the appended text has to read as a separate paragraph to a model that is about to be handed the whole thing as one string, and the shapes must agree about that, because a template is free to join an array’s text parts with nothing between them.