Add the ability to close and reopen the chat window via SDK

Editor assessment

The subject states the capability concretely, the chat window can be closed and reopened through the SDK, and feat is right, which makes the omission sharper: no body documents the new API calls, their names, or their behavior, the contract SDK consumers exist to read. No scope, no footers. The trailing pull-request number is the conventional squash-merge marker. Hygiene is good: imperative shape, no trailing period, no junk, moderate length. Clear at the headline and missing exactly the interface documentation that would teach.