fix: integration test for #9011 (#9166)

Adds an integration test for the new behavior introduced in
https://github.com/openai/codex/pull/9011. The work to create the test
setup was substantial enough that I thought it merited a separate PR.

This integration test spawns `codex` in TUI mode, which requires
spawning a PTY to run successfully, so I had to introduce quite a bit of
scaffolding in `run_codex_cli()`. I was surprised to discover that we
have not done this in our codebase before, so perhaps this should get
moved to a common location so it can be reused.

The test itself verifies that a malformed `rules` in `$CODEX_HOME`
prints a human-readable error message and exits nonzero.

codex-rs/tui/tests/all.rs

@@ -3,4 +3,7 @@
 #[cfg(feature = "vt100-tests")]
 mod test_backend;
 
+#[allow(unused_imports)]
+use codex_cli as _; // Keep dev-dep for cargo-shear; tests spawn the codex binary.
+
 mod suite;