code-mode: move to hosted mode by default (#31500)

## Summary

  - Promote code_mode_host to stable and enable it by default.
- Preserve features.code_mode_host = false as an opt-out to the
in-process runtime.
  - Run core code-mode tests through the standalone host.
  - Keep explicit coverage for missing-host failures.

codex-rs/features/src/tests.rs

@@ -123,6 +123,16 @@ fn code_mode_only_requires_code_mode() {
     assert_eq!(features.enabled(Feature::CodeMode), true);
 }
 
+#[test]
+fn code_mode_host_is_stable_and_enabled_by_default() {
+    assert_eq!(Feature::CodeModeHost.stage(), Stage::Stable);
+    assert_eq!(Feature::CodeModeHost.default_enabled(), true);
+    assert_eq!(
+        feature_for_key("code_mode_host"),
+        Some(Feature::CodeModeHost)
+    );
+}
+
 #[test]
 fn guardian_approval_is_stable_and_enabled_by_default() {
     let spec = Feature::GuardianApproval.info();