## Summary
This PR fixes restricted filesystem permission profiles so Codex's
runtime-managed helper executables remain readable without requiring
explicit user configuration.
- add implicit readable roots for the configured `zsh` helper path and
the main execve wrapper
- allowlist the shared `$CODEX_HOME/tmp/arg0` root when the execve
wrapper lives there, so session-specific helper paths keep working
- dedupe injected paths and avoid adding duplicate read entries to the
sandbox policy
- add regression coverage for restricted read mode with helper
executable overrides
## Testing
before this change: got this error when executing a shell command via
zsh fork:
```
"sandbox error: sandbox denied exec error, exit code: 127, stdout: , stderr: /etc/zprofile:11: operation not permitted: /usr/libexec/path_helper\nzsh:1: operation not permitted: .codex/skills/proxy-a/scripts/fetch_example.sh\n"
```
saw this change went away after this change, meaning the readable roots
and injected correctly.
## Why
PR #13783 moved the `codex.rs` unit tests into `codex_tests.rs`. This
applies the same extraction pattern across the rest of `codex-rs/core`
so the production modules stay focused on runtime code instead of large
inline test blocks.
Keeping the tests in sibling files also makes follow-up edits easier to
review because product changes no longer have to share a file with
hundreds or thousands of lines of test scaffolding.
## What changed
- replaced each inline `mod tests { ... }` in `codex-rs/core/src/**`
with a path-based module declaration
- moved each extracted unit test module into a sibling `*_tests.rs`
file, using `mod_tests.rs` for `mod.rs` modules
- preserved the existing `cfg(...)` guards and module-local structure so
the refactor remains structural rather than behavioral
## Testing
- `cargo test -p codex-core --lib` (`1653 passed; 0 failed; 5 ignored`)
- `just fix -p codex-core`
- `cargo fmt --check`
- `cargo shear`
Celia Chen