## 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`
We already support reading from `config.toml` through a symlink, but the
code was not properly handling updates to a symlinked config file. This
PR generalizes safe symlink-chain resolution and atomic writes into
path_utils, updating all config write paths to use the shared logic
(including set_default_oss_provider, which previously didn't use the
common path), and adds tests for symlink chains and cycles.
This resolves#6646.
Notes:
* Symlink cycles or resolution failures replace the top-level symlink
with a real file.
* Shared config write path now handles symlinks consistently across
edits, defaults, and empty-user-layer creation.
This PR was inspired by https://github.com/openai/codex/pull/9437, which
was contributed by @ryoppippi
Michael BolinandGitHub
jif-oaiandGitHub
Eric TrautandGitHub