Preserve precedence across feature requirement aliases (#42863)

## Why

Requirements layers may use either `features` or its `feature_requirements`
alias. Treating those names as separate tables prevents feature settings from
merging consistently across layers.

## What changed

Normalize `feature_requirements` to `features` after validating each layer so
mixed aliases share one merge path and retain layer precedence.

## Testing

Add coverage for both alias orderings across low- and high-priority layers.

GitOrigin-RevId: e23ad3e0853b8b49ca10447875f33706de39bcfe

codex-rs/config/src/requirements_layers/layer.rs

@@ -102,6 +102,13 @@ impl ComposableRequirementsLayer {
             (regular_toml, requirements)
         };
 
+        // Merge aliases under one key after validating each individual layer.
+        if let Some(table) = regular_toml.as_table_mut()
+            && let Some(features) = table.remove("feature_requirements")
+        {
+            table.insert("features".to_string(), features);
+        }
+
         // Hostname lookup is configuration-driven and may block on DNS, so only
         // resolve it when this layer contains hostname-based sandbox selectors.
         let hostname = requirements

codex-rs/config/src/requirements_layers/stack_tests.rs

@@ -469,6 +469,35 @@ approval_mode = "approve"
     );
 }
 
+#[test]
+fn feature_aliases_merge_with_layer_precedence() {
+    for (low_key, high_key) in [
+        ("features", "feature_requirements"),
+        ("feature_requirements", "features"),
+    ] {
+        let composed = compose(vec![
+            layer(
+                "req_low",
+                "Low",
+                &format!("[{low_key}]\nchronicle = true\nshell_snapshot = false"),
+            ),
+            layer(
+                "req_high",
+                "High",
+                &format!("[{high_key}]\nchronicle = false\napps = false"),
+            ),
+        ])
+        .expect("compose mixed feature aliases");
+
+        assert_eq!(
+            composed,
+            Some(expected_requirements(
+                "[features]\nchronicle = false\nshell_snapshot = false\napps = false"
+            ))
+        );
+    }
+}
+
 #[test]
 fn merged_table_source_is_composite_in_priority_order() {
     let high_source = RequirementSource::EnterpriseManaged {