feat: Constrain values for approval_policy (#7778)

Constrain `approval_policy` through new `admin_policy` config.

This PR will:
1. Add a `admin_policy` section to config, with a single field (for now)
`allowed_approval_policies`. This list constrains the set of
user-settable `approval_policy`s.
2. Introduce a new `Constrained<T>` type, which combines a current value
and a validator function. The validator function ensures disallowed
values are not set.
3. Change the type of `approval_policy` on `Config` and
`SessionConfiguration` from `AskForApproval` to
`Constrained<AskForApproval>`. The validator function is set by the
values passed into `allowed_approval_policies`.
4. `GenericDisplayRow`: add a `disabled_reason: Option<String>`. When
set, it disables selection of the value and indicates as such in the
menu. This also makes it unselectable with arrow keys or numbers. This
is used in the `/approvals` menu.

Follow ups are:
1. Do the same thing to `sandbox_policy`.
2. Propagate the allowed set of values through app-server for the
extension (though already this should prevent app-server from setting
this values, it's just that we want to disable UI elements that are
unsettable).

Happy to split this PR up if you prefer, into the logical numbered areas
above. Especially if there are parts we want to gavel on separately
(e.g. admin_policy).

Disabled full access:
<img width="1680" height="380" alt="image"
src="https://github.com/user-attachments/assets/1fb61c8c-1fcb-4dc4-8355-2293edb52ba0"
/>

Disabled `--yolo` on startup:
<img width="749" height="76" alt="image"
src="https://github.com/user-attachments/assets/0a1211a0-6eb1-40d6-a1d7-439c41e94ddb"
/>

CODEX-4087

codex-rs/tui2/src/chatwidget.rs

@@ -2556,7 +2556,7 @@ impl ChatWidget {
 
     /// Open a popup to choose the approvals mode (ask for approval policy + sandbox policy).
     pub(crate) fn open_approvals_popup(&mut self) {
-        let current_approval = self.config.approval_policy;
+        let current_approval = self.config.approval_policy.value();
         let current_sandbox = self.config.sandbox_policy.clone();
         let mut items: Vec<SelectionItem> = Vec::new();
         let presets: Vec<ApprovalPreset> = builtin_approval_presets();
@@ -2954,7 +2954,9 @@ impl ChatWidget {
 
     /// Set the approval policy in the widget's config copy.
     pub(crate) fn set_approval_policy(&mut self, policy: AskForApproval) {
-        self.config.approval_policy = policy;
+        if let Err(err) = self.config.approval_policy.set(policy) {
+            tracing::warn!(%err, "failed to set approval_policy on chat config");
+        }
     }
 
     /// Set the sandbox policy in the widget's config copy.