Problem: An OSC 52 sequence from a :terminal job passes the decoded
payload to the clipboard provider as a single list item. Command-line
providers (pbcopy, xclip, ...) receive it with channel semantics, where
a newline inside an item is sent as NUL (:h chansend()), so multiline
copies arrive with NUL bytes instead of newlines.
Solution: Split the payload on newlines into a proper list of lines.
A trailing newline yields a final empty item, which chansend() turns
back into a newline, so payloads round-trip exactly.