Improve codeowners validation errors in file edit

What does this MR do?

An iterative change, since much of this will be updated in !34471 (merged), however still worth doing. I've updated the logic to better recognize file creation (which was falling back onto legacy codeowner checks which weren't properly handling the response message text) and added code to strip the \n; I've future work to better approach word wrap formatting for error display in the CLI (which is why we have those in there in the first place..) but this addresses the immediate issue. While working this, I also updated the specs to use actual codeowner checks of the incoming files, to prove the message is formatted correctly.

Closes #221199 (closed)