feat(requirements): support allowed_approval_reviewers (#16701)

## Description

Add requirements.toml support for `allowed_approvals_reviewers =
["user", "guardian_subagent"]`, so admins can now restrict the use of
guardian mode.

Note: If a user sets a reviewer that isn’t allowed by requirements.toml,
config loading falls back to the first allowed reviewer and emits a
startup warning.

The table below describes the possible admin controls.
| Admin intent | `requirements.toml` | User `config.toml` | End result |
|---|---|---|---|
| Leave Guardian optional | omit `allowed_approvals_reviewers` or set
`["user", "guardian_subagent"]` | user chooses `approvals_reviewer =
"user"` or `"guardian_subagent"` | Guardian off for `user`, on for
`guardian_subagent` + `approval_policy = "on-request"` |
| Force Guardian off | `allowed_approvals_reviewers = ["user"]` | any
user value | Effective reviewer is `user`; Guardian off |
| Force Guardian on | `allowed_approvals_reviewers =
["guardian_subagent"]` and usually `allowed_approval_policies =
["on-request"]` | any user reviewer value; user should also have
`approval_policy = "on-request"` unless policy is forced | Effective
reviewer is `guardian_subagent`; Guardian on when effective approval
policy is `on-request` |
| Allow both, but default to manual if user does nothing |
`allowed_approvals_reviewers = ["user", "guardian_subagent"]` | omit
`approvals_reviewer` | Effective reviewer is `user`; Guardian off |
| Allow both, and user explicitly opts into Guardian |
`allowed_approvals_reviewers = ["user", "guardian_subagent"]` |
`approvals_reviewer = "guardian_subagent"` and `approval_policy =
"on-request"` | Guardian on |
| Invalid admin config | `allowed_approvals_reviewers = []` | anything |
Config load error |

codex-rs/core/src/codex_tests.rs

@@ -2216,7 +2216,7 @@ fn text_block(s: &str) -> serde_json::Value {
 }
 
 async fn build_test_config(codex_home: &Path) -> Config {
-    ConfigBuilder::default()
+    ConfigBuilder::without_managed_config_for_tests()
         .codex_home(codex_home.to_path_buf())
         .build()
         .await