Intermittent test panic in `cancelled_turn_with_usage_emits_notification_before_response`

[stevelr]

Found while trying to reproduce #4939

Test cancelled_turn_with_usage_emits_notification_before_response fails 1-4 out of 20 times when running fake_llm tests.

Describe the bug

diagnosis written by gpt-5.6-sol

The test does:

let c_id = h.send("session/cancel", ...).await;
let _ = gate_tx.send(());

h.send() only writes the cancel request to stdin; it does not prove the agent
processed it. The test may release the gated HTTP response first. That response
contains the deliberate “no canned response” error, so session/prompt returns a
JSON-RPC error and:

v["result"]["stopReason"]

is Null.

Suggested fix: wait for the cancel acknowledgement before releasing the HTTP
gate, while retaining any usage or prompt frames received before that
acknowledgement:

let c_id = h.send("session/cancel", json!({"sessionId": sid})).await;
let (frames_before_cancel_ack, _) =
recv_until_with_drain(&mut h, |v| v["id"] == json!(c_id)).await;

let _ = gate_tx.send(());

The collected frames then need to be included in the test’s ordering assertions.

Steps to reproduce

  for i in {1..20}; do
    echo "attempt $i"
    cargo test --release -p buzz-agent --test fake_llm
  done

Look for

thread 'cancelled_turn_with_usage_emits_notification_before_response' (632197)
  panicked at crates/buzz-agent/tests/fake_llm.rs:1370:13:
  assertion `left == right` failed: turn must end with stopReason: cancelled
    left: Null
   right: "cancelled"
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


  failures:
      cancelled_turn_with_usage_emits_notification_before_response

In 20 test runs, this failure occurs 1-4 times.

Version and platform

buzz-desktop 0.5.5
x86_64-linux