Move shared_buffers into BufferStore (#17020)

This also updates the SSH protocol (but not yet collab) to more closely
track which buffers are open on the client.

Release Notes:

- N/A

crates/proto/src/proto.rs

@@ -411,7 +411,8 @@ messages!(
     (AddWorktree, Foreground),
     (AddWorktreeResponse, Foreground),
     (FindSearchCandidates, Background),
-    (FindSearchCandidatesResponse, Background)
+    (FindSearchCandidatesResponse, Background),
+    (CloseBuffer, Foreground)
 );
 
 request_messages!(