You know that feeling when you’re about to finish a particular thought and suddenly someone else, a friend or a stranger, finishes your sentence better than you possibly could have? Is there a word for that in English, or in any language?
When that happens, it is common to say "you took the words right out of my mouth" or "you read my mind"
I think the word you're looking for is co-narration.
I'm currently taking a communications class, and that is the word used when people who are in a relationship (close or otherwise) finish each other's sentences because they have enough knowledge to infer what the person is trying to say.
However consider adding an adjective.
E.g. Awesome dude.
In any language? In France we call that person "sauveur" (=lifesaver).
So, A "life saver" is often used to address someone who's been such a great help for you, including finishing things better than you could possibly have.
Someone who brings the wrong kind of closure could be called a terminator.
Since an already existing answer seems unlikely, may I humbly suggest the term clauseguard?