Add an acknowledgment response to subscriptions/listen

### Where was this agreed?

https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/2905

### Description

`subscriptions/listen` is currently the only request in the protocol with no response object. Decision at the Jun 10 Core+SDK maintainer call was to add an empty ack response. Touches `schema/draft/schema.ts` and `basic/patterns/subscriptions.mdx`.