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/config/mod.rs

@@ -689,6 +689,11 @@ impl ConfigBuilder {
             config_layer_stack,
         )
     }
+
+    #[cfg(test)]
+    pub(crate) fn without_managed_config_for_tests() -> Self {
+        Self::default().loader_overrides(LoaderOverrides::without_managed_config_for_tests())
+    }
 }
 
 impl Config {
@@ -2060,6 +2065,7 @@ impl Config {
         // Config.
         let ConfigRequirements {
             approval_policy: mut constrained_approval_policy,
+            approvals_reviewer: mut constrained_approvals_reviewer,
             sandbox_policy: mut constrained_sandbox_policy,
             web_search_mode: mut constrained_web_search_mode,
             feature_requirements,
@@ -2308,10 +2314,22 @@ impl Config {
             );
             approval_policy = constrained_approval_policy.value();
         }
-        let approvals_reviewer = approvals_reviewer_override
+        let approvals_reviewer_was_explicit = approvals_reviewer_override.is_some()
+            || config_profile.approvals_reviewer.is_some()
+            || cfg.approvals_reviewer.is_some();
+        let mut approvals_reviewer = approvals_reviewer_override
             .or(config_profile.approvals_reviewer)
             .or(cfg.approvals_reviewer)
             .unwrap_or(ApprovalsReviewer::User);
+        if !approvals_reviewer_was_explicit
+            && let Err(err) = constrained_approvals_reviewer.can_set(&approvals_reviewer)
+        {
+            tracing::warn!(
+                error = %err,
+                "default approvals reviewer is disallowed by requirements; falling back to required default"
+            );
+            approvals_reviewer = constrained_approvals_reviewer.value();
+        }
         let web_search_mode = resolve_web_search_mode(&cfg, &config_profile, &features)
             .unwrap_or(WebSearchMode::Cached);
         let web_search_config = resolve_web_search_config(&cfg, &config_profile);
@@ -2554,6 +2572,12 @@ impl Config {
             &mut constrained_approval_policy,
             &mut startup_warnings,
         )?;
+        apply_requirement_constrained_value(
+            "approvals_reviewer",
+            approvals_reviewer,
+            &mut constrained_approvals_reviewer,
+            &mut startup_warnings,
+        )?;
         apply_requirement_constrained_value(
             "sandbox_mode",
             sandbox_policy,
@@ -2639,7 +2663,7 @@ impl Config {
                 windows_sandbox_mode,
                 windows_sandbox_private_desktop,
             },
-            approvals_reviewer,
+            approvals_reviewer: constrained_approvals_reviewer.value(),
             enforce_residency: enforce_residency.value,
             notify: cfg.notify,
             user_instructions,