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/Cargo.lock

@@ -250,6 +250,28 @@ dependencies = [
  "pin-project-lite",
 ]
 
+[[package]]
+name = "async-stream"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b5a71a6f37880a80d1d7f19efd781e4b5de42c88f0722cc13bcb6cc2cfe8476"
+dependencies = [
+ "async-stream-impl",
+ "futures-core",
+ "pin-project-lite",
+]
+
+[[package]]
+name = "async-stream-impl"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7c24de15d275a1ecfd47a380fb4d5ec9bfe0933f309ed5e705b775596a3574d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.104",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.88"
@@ -654,6 +676,7 @@ dependencies = [
  "thiserror 2.0.12",
  "time",
  "tokio",
+ "tokio-test",
  "tokio-util",
  "toml 0.9.1",
  "tracing",
@@ -4516,6 +4539,30 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tokio-stream"
+version = "0.1.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eca58d7bba4a75707817a2c44174253f9236b2d5fbd055602e9d5c07c139a047"
+dependencies = [
+ "futures-core",
+ "pin-project-lite",
+ "tokio",
+]
+
+[[package]]
+name = "tokio-test"
+version = "0.4.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2468baabc3311435b55dd935f702f42cd1b8abb7e754fb7dfb16bd36aa88f9f7"
+dependencies = [
+ "async-stream",
+ "bytes",
+ "futures-core",
+ "tokio",
+ "tokio-stream",
+]
+
 [[package]]
 name = "tokio-util"
 version = "0.7.15"