bxctl: escape frame errors; say the outcome is unknown after a timeout

From the independent review of task 23. serde quotes a bad frame's text after
decoding, so a compromised peer could put escape sequences in it. A timed-out
admin request now says whether brokerd acted is unknown, since it may have.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/bxctl/tests/admin_timeout.rs

@@ -31,7 +31,11 @@ fn a_silent_brokerd_is_an_error_after_the_timeout() {
         Duration::from_millis(200),
     );
     let took = started.elapsed();
-    assert!(got.is_err(), "{got:?}");
+    let text = got.expect_err("a silent brokerd is an error").to_string();
+    assert!(
+        text.contains("whether it acted is unknown"),
+        "brokerd may have approved and run the call before going quiet: {text}"
+    );
     assert!(took < Duration::from_millis(1_500), "waited {took:?}");
     held.join().unwrap();
     let _ = std::fs::remove_dir_all(&dir);