bxctl: chat_approvals' helper accepts a broken pipe on stdin

json_prints_only_json_and_never_asks_brokerd failed once under load: the helper wrote the typed
input to a `bxctl chat --json` that never reads stdin and had already exited, and unwrapped the
broken pipe. A broken pipe on that write is now accepted; any other error still fails the test,
and a test whose input must be read checks the output.

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

crates/bxctl/tests/chat_approvals.rs

@@ -398,8 +398,15 @@ fn chat(loopd: &Path, brokerd: &Path, extra: &[&str], typed: &str) -> std::proce
         .spawn()
         .unwrap();
     {
+        // A `bxctl` that never reads stdin (with `--json`, or with no approval to answer) may
+        // exit before the write: a broken pipe then is not an error. Anything that must be read
+        // is checked by the test's own assertions on the output.
         let mut stdin = child.stdin.take().unwrap();
-        stdin.write_all(typed.as_bytes()).unwrap();
+        match stdin.write_all(typed.as_bytes()) {
+            Ok(()) => {}
+            Err(e) if e.kind() == std::io::ErrorKind::BrokenPipe => {}
+            Err(e) => panic!("writing to bxctl's stdin: {e}"),
+        }
     }
     child.wait_with_output().unwrap()
 }