Enable unavailable dummy tools by default (#19459)

## Summary
- Mark `unavailable_dummy_tools` as a stable feature and enable it by
default
- Update the feature registry test to match the new default state

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

codex-rs/features/src/tests.rs

@@ -142,15 +142,6 @@ fn browser_controls_are_stable_and_enabled_by_default() {
     assert_eq!(feature_for_key("computer_use"), Some(Feature::ComputerUse));
 }
 
-#[test]
-fn unavailable_dummy_tools_is_under_development_and_disabled_by_default() {
-    assert_eq!(
-        Feature::UnavailableDummyTools.stage(),
-        Stage::UnderDevelopment
-    );
-    assert_eq!(Feature::UnavailableDummyTools.default_enabled(), false);
-}
-
 #[test]
 fn general_analytics_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::GeneralAnalytics.stage(), Stage::Stable);