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/app-server-protocol/src/protocol/v2.rs

@@ -850,6 +850,8 @@ pub struct ConfigReadResponse {
 pub struct ConfigRequirements {
     #[experimental(nested)]
     pub allowed_approval_policies: Option<Vec<AskForApproval>>,
+    #[experimental("configRequirements/read.allowedApprovalsReviewers")]
+    pub allowed_approvals_reviewers: Option<Vec<ApprovalsReviewer>>,
     pub allowed_sandbox_modes: Option<Vec<SandboxMode>>,
     pub allowed_web_search_modes: Option<Vec<WebSearchMode>>,
     pub feature_requirements: Option<BTreeMap<String, bool>>,
@@ -7315,6 +7317,7 @@ mod tests {
                     request_permissions: false,
                     mcp_elicitations: false,
                 }]),
+                allowed_approvals_reviewers: None,
                 allowed_sandbox_modes: None,
                 allowed_web_search_modes: None,
                 feature_requirements: None,