Mark client as disconnected on connection close/error

Instead of leaving onClose/onError as no-ops (which would leave the
client in a stale 'connected' state), transition to 'disconnected'
so callers fail fast or can re-start cleanly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

nodejs/src/client.ts

@@ -1410,11 +1410,11 @@ export class CopilotClient {
         );
 
         this.connection.onClose(() => {
-            // no-op
+            this.state = "disconnected";
         });
 
         this.connection.onError((_error) => {
-            // no-op
+            this.state = "disconnected";
         });
     }