Handle expanded command results in Rust tests

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

Author: stephentoub

rust/tests/e2e/commands.rs

@@ -120,6 +120,7 @@ async fn session_commands_invoke_known_builtin_returns_expected_result() {
                         assert!(!prompt.prompt.trim().is_empty());
                     }
                     SlashCommandInvocationResult::Completed(_) => {}
+                    unexpected => panic!("unexpected command result: {unexpected:?}"),
                 }
 
                 session.disconnect().await.expect("disconnect session");