add a ton of tests

codex-rs/utils/pty/README.md

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+# codex-utils-pty parity notes
+
+The tests in `utils/pty/src/lib.rs` compare PTY-backed sessions with the piped
+fallback to ensure the shared behaviors stay aligned. Some behavior differences
+are inherent to PTY semantics, so the tests normalize or explicitly allow them.
+
+Known differences
+
+- TTY line discipline: PTY-backed children run with a terminal line discipline
+  (canonical input, echo, signal generation). Piped fallback uses plain pipes,
+  so input is raw and no echo or line editing occurs.
+- Line endings: PTY output may translate LF to CRLF (for example when `ONLCR` is
+  enabled). Piped output preserves what the program writes.
+- Stdout/stderr interleaving: PTY output is a single stream with ordering
+  preserved by the terminal. Piped fallback merges stdout/stderr from separate
+  readers, so relative ordering between the streams is not guaranteed.
+- Terminal features: PTY sessions support terminal-only behaviors (window size,
+  control sequences). Piped fallback does not emulate these features.
+- Signals: PTY sessions can receive terminal-generated signals (such as Ctrl+C).
+  Piped fallback does not provide that terminal-level signaling.