(MacOS) Load config requirements from MDM (#8743)

Load managed requirements from MDM key `requirements_toml_base64`.

Tested on my Mac (using `defaults` to set the preference, though this
would be set by MDM in production):

```
➜  codex git:(gt/mdm-requirements) defaults read com.openai.codex requirements_toml_base64 | base64 -d
allowed_approval_policies = ["on-request"]

➜  codex git:(gt/mdm-requirements) just c --yolo
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.26s
     Running `target/debug/codex --yolo`
Error loading configuration: value `Never` is not in the allowed set [OnRequest]
error: Recipe `codex` failed on line 11 with exit code 1

➜  codex git:(gt/mdm-requirements) defaults delete com.openai.codex requirements_toml_base64

➜  codex git:(gt/mdm-requirements) just c --yolo
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.24s
     Running `target/debug/codex --yolo`
╭──────────────────────────────────────────────────────────╮
│ >_ OpenAI Codex (v0.0.0)                                 │
│                                                          │
│ model:     codex-auto-balanced medium   /model to change │
│ directory: ~/code/codex/codex-rs                         │
╰──────────────────────────────────────────────────────────╯

  Tip: Start a fresh idea with /new; the previous session stays in history.
```

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

@@ -12,11 +12,14 @@ use std::collections::HashMap;
 use std::path::PathBuf;
 use toml::Value as TomlValue;
 
+/// LoaderOverrides overrides managed configuration inputs (primarily for tests).
 #[derive(Debug, Default, Clone)]
 pub struct LoaderOverrides {
     pub managed_config_path: Option<PathBuf>,
+    //TODO(gt): Add a macos_ prefix to this field and remove the target_os check.
     #[cfg(target_os = "macos")]
     pub managed_preferences_base64: Option<String>,
+    pub macos_managed_config_requirements_base64: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq)]