Warn for structured feature toggles (#27076)

## Summary
Startup warnings for under-development features only recognized bare
boolean toggles like `features.foo = true`. An upcoming feature will use
table-format config, so `features.foo = { enabled = true, ... }` needs
to count as an explicit opt-in too.

This updates the warning predicate to recognize structured tables with
`enabled = true`, while leaving tables without that field unwarned.

## Testing
- `just fmt`
- `just test -p codex-features
unstable_warning_event_mentions_enabled_structured_under_development_feature`

codex-rs/features/src/lib.rs

@@ -1281,7 +1281,13 @@ pub fn unstable_features_warning_event(
     let mut under_development_feature_keys = Vec::new();
     if let Some(table) = effective_features {
         for (key, value) in table {
-            if value.as_bool() != Some(true) {
+            let is_enabled = value.as_bool() == Some(true)
+                || value
+                    .as_table()
+                    .and_then(|table| table.get("enabled"))
+                    .and_then(toml::Value::as_bool)
+                    == Some(true);
+            if !is_enabled {
                 continue;
             }
             let Some(spec) = FEATURES.iter().find(|spec| spec.key == key.as_str()) else {
@@ -1300,6 +1306,7 @@ pub fn unstable_features_warning_event(
         return None;
     }
 
+    under_development_feature_keys.sort();
     let under_development_feature_keys = under_development_feature_keys.join(", ");
     let message = format!(
         "Under-development features enabled: {under_development_feature_keys}. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in {config_path}."