Close (remove) a document from a multi-document session.

Remove document_id from the session's open documents (the tab close button). If it's the focused document, focus another open document — next_focus if it's still open (so the client can pick the adjacent tab), else the next remaining one; closing the last document leaves the session empty. Returns the updated document roster + the now-focused document's HTML.

The close takes effect immediately and is persisted, so a later reconnect or session restore won't bring the closed document back. Persistence is best-effort — a failure is logged, not fatal (the close still holds for the active session).