Add exec policy TOML representation (#10026)

We'd like to represent these in `requirements.toml`. This just adds the
representation and the tests, doesn't wire it up anywhere yet.

codex-rs/core/src/config_loader/mod.rs

@@ -6,6 +6,8 @@ mod layer_io;
 mod macos;
 mod merge;
 mod overrides;
+#[cfg(test)]
+mod requirements_exec_policy;
 mod state;
 
 #[cfg(test)]