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/tui/src/bottom_pane/selection_popup_common.rs

@@ -20,6 +20,7 @@ pub(crate) struct GenericDisplayRow {
     pub display_shortcut: Option<KeyBinding>,
     pub match_indices: Option<Vec<usize>>, // indices to bold (char positions)
     pub description: Option<String>,       // optional grey text after the name
+    pub disabled_reason: Option<String>,   // optional disabled message
     pub wrap_indent: Option<usize>,        // optional indent for wrapped lines
 }
 
@@ -37,7 +38,13 @@ fn compute_desc_col(
         .iter()
         .enumerate()
         .filter(|(i, _)| visible_range.contains(i))
-        .map(|(_, r)| Line::from(r.name.clone()).width())
+        .map(|(_, r)| {
+            let mut spans: Vec<Span> = vec![r.name.clone().into()];
+            if r.disabled_reason.is_some() {
+                spans.push(" (disabled)".dim());
+            }
+            Line::from(spans).width()
+        })
         .max()
         .unwrap_or(0);
     let mut desc_col = max_name_width.saturating_add(2);
@@ -51,7 +58,7 @@ fn compute_desc_col(
 fn wrap_indent(row: &GenericDisplayRow, desc_col: usize, max_width: u16) -> usize {
     let max_indent = max_width.saturating_sub(1) as usize;
     let indent = row.wrap_indent.unwrap_or_else(|| {
-        if row.description.is_some() {
+        if row.description.is_some() || row.disabled_reason.is_some() {
             desc_col
         } else {
             0
@@ -64,10 +71,16 @@ fn wrap_indent(row: &GenericDisplayRow, desc_col: usize, max_width: u16) -> usiz
 /// at `desc_col`. Applies fuzzy-match bolding when indices are present and
 /// dims the description.
 fn build_full_line(row: &GenericDisplayRow, desc_col: usize) -> Line<'static> {
+    let combined_description = match (&row.description, &row.disabled_reason) {
+        (Some(desc), Some(reason)) => Some(format!("{desc} (disabled: {reason})")),
+        (Some(desc), None) => Some(desc.clone()),
+        (None, Some(reason)) => Some(format!("disabled: {reason}")),
+        (None, None) => None,
+    };
+
     // Enforce single-line name: allow at most desc_col - 2 cells for name,
     // reserving two spaces before the description column.
-    let name_limit = row
-        .description
+    let name_limit = combined_description
         .as_ref()
         .map(|_| desc_col.saturating_sub(2))
         .unwrap_or(usize::MAX);
@@ -113,6 +126,10 @@ fn build_full_line(row: &GenericDisplayRow, desc_col: usize) -> Line<'static> {
         name_spans.push("…".into());
     }
 
+    if row.disabled_reason.is_some() {
+        name_spans.push(" (disabled)".dim());
+    }
+
     let this_name_width = Line::from(name_spans.clone()).width();
     let mut full_spans: Vec<Span> = name_spans;
     if let Some(display_shortcut) = row.display_shortcut {
@@ -120,7 +137,7 @@ fn build_full_line(row: &GenericDisplayRow, desc_col: usize) -> Line<'static> {
         full_spans.push(display_shortcut.into());
         full_spans.push(")".into());
     }
-    if let Some(desc) = row.description.as_ref() {
+    if let Some(desc) = combined_description.as_ref() {
         let gap = desc_col.saturating_sub(this_name_width);
         if gap > 0 {
             full_spans.push(" ".repeat(gap).into());