## Summary
- replace the unsubscribe-during-turn test's sleep/polling flow with a
gated streaming SSE response
- add request-count notification support to the streaming SSE test
server so the test can wait for the in-flight Responses request
deterministically
## Scope
- codex-rs/app-server/tests/suite/v2/thread_unsubscribe.rs
- codex-rs/core/tests/common/streaming_sse.rs
## Validation
- Not run locally; this is a narrow extraction from the prior CI-green
branch.
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Co-authored-by: Codex <noreply@openai.com>
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