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/tests.rs

@@ -712,3 +712,34 @@ fn unstable_warning_event_only_mentions_enabled_under_development_features() {
     assert!(!message.contains("personality"));
     assert!(message.contains("/tmp/config.toml"));
 }
+
+#[test]
+fn unstable_warning_event_mentions_enabled_structured_under_development_feature() {
+    let configured_features: Table = toml::from_str(
+        r#"
+multi_agent_v2 = { enabled = true, tool_namespace = "agents" }
+code_mode = true
+"#,
+    )
+    .expect("features table should deserialize");
+
+    let mut features = Features::with_defaults();
+    features.enable(Feature::MultiAgentV2);
+    features.enable(Feature::CodeMode);
+
+    let warning = unstable_features_warning_event(
+        Some(&configured_features),
+        /*suppress_unstable_features_warning*/ false,
+        &features,
+        "/tmp/config.toml",
+    )
+    .expect("warning event");
+
+    let EventMsg::Warning(WarningEvent { message }) = warning.msg else {
+        panic!("expected warning event");
+    };
+    assert_eq!(
+        "Under-development features enabled: code_mode, multi_agent_v2. Under-development features are incomplete and may behave unpredictably. To suppress this warning, set `suppress_unstable_features_warning = true` in /tmp/config.toml.".to_string(),
+        message
+    );
+}