chore: simplify loading of Mac-specific logic in config_loader (#8248)

Over in `config_loader/macos.rs`, we were doing this complicated `mod`
thing to expose one version of `load_managed_admin_config_layer()` for
Mac:


580c59aa9a/codex-rs/core/src/config_loader/macos.rs (L4-L5)

While exposing a trivial implementation for non-Mac:


580c59aa9a/codex-rs/core/src/config_loader/macos.rs (L110-L117)

That was being used like this:


580c59aa9a/codex-rs/core/src/config_loader/layer_io.rs (L47-L48)

This PR simplifies that callsite in `layer_io.rs` to just be:

```rust
    #[cfg(not(target_os = "macos"))]
    let managed_preferences = None;
```

And updates `config_loader/mod.rs` so we only pull in `macos.rs` on Mac:

```rust
#[cfg(target_os = "macos")]
mod macos;
```

This simplifies `macos.rs` considerably, though it looks like a big
change because everything gets unindented and reformatted because we can
drop the whole `mod native` thing now.




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* #8251
* #8249
* __->__ #8248

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

@@ -1,5 +1,6 @@
 mod fingerprint;
 mod layer_io;
+#[cfg(target_os = "macos")]
 mod macos;
 mod merge;
 mod overrides;