(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/mod.rs

@@ -2050,6 +2050,7 @@ trust_level = "trusted"
             managed_config_path: Some(managed_path.clone()),
             #[cfg(target_os = "macos")]
             managed_preferences_base64: None,
+            macos_managed_config_requirements_base64: None,
         };
 
         let cwd = AbsolutePathBuf::try_from(codex_home.path())?;
@@ -2170,6 +2171,7 @@ trust_level = "trusted"
             managed_config_path: Some(managed_path),
             #[cfg(target_os = "macos")]
             managed_preferences_base64: None,
+            macos_managed_config_requirements_base64: None,
         };
 
         let cwd = AbsolutePathBuf::try_from(codex_home.path())?;