Add SSE Response parser tests (#1541)

## Summary
- add `tokio-test` dev dependency
- implement response stream parsing unit tests

## Testing
- `cargo clippy -p codex-core --tests -- -D warnings`
- `cargo test -p codex-core -- --nocapture`

------
https://chatgpt.com/codex/tasks/task_i_687163f3b2208321a6ce2adbef3fbc06

codex-rs/core/Cargo.toml

@@ -64,4 +64,5 @@ maplit = "1.0.2"
 predicates = "3"
 pretty_assertions = "1.4.1"
 tempfile = "3"
+tokio-test = "0.4"
 wiremock = "0.6"