test(agent): wait for cancel acknowledgement

[kiranmagic7]

Summary

Wait for the session/cancel acknowledgement before releasing the gated round-two LLM response, then include frames received before that acknowledgement in the existing ordering assertions.

Writing the cancel request to stdin did not prove the agent had processed it. The test could therefore release the deliberate error response first and observe a null stopReason instead of cancelled.

Related issue

Fixes #4945. No duplicate issue or PR was found in the current open or historical searches.

Testing

• Unmodified main: the affected test failed in 16 of 100 full fake_llm suite runs.
• Patched: the affected test failed in 0 of 100 full-suite runs. The entire suite passed in 82 runs; the other 18 failed only in unrelated pre-existing steering tests.
• bin/cargo test -p buzz-agent --test fake_llm cancelled_turn_with_usage_emits_notification_before_response -- --exact
• bin/cargo test -p buzz-agent
• bin/just ci
• Screenshots: N/A (test-only change).

[wesbillman]

Awesome, thanks for this!