[Codex] Add browser use external feature flag (#20245)

## Summary

- Adds a separate feature control for external-browser Browser Use
integrations.
- Registers `browser_use_external` as a stable, default-enabled
requirements-owned feature key.
- Updates feature registry tests and regenerates the config schema.

Codex validation:
- `cargo fmt -- --config imports_granularity=Item`
- `cargo run -p codex-core --bin codex-write-config-schema`
- `cargo test -p codex-features`

## Addendum

This gives enterprise policy a coarse control for Browser Use outside
the Codex-managed in-app browser. The existing `browser_use` feature is
the Browser Use control, while `browser_use_external` can gate
extension/native integrations for external browsers as that surface
grows

codex-rs/features/src/tests.rs

@@ -157,6 +157,13 @@ fn browser_controls_are_stable_and_enabled_by_default() {
     assert_eq!(Feature::BrowserUse.default_enabled(), true);
     assert_eq!(feature_for_key("browser_use"), Some(Feature::BrowserUse));
 
+    assert_eq!(Feature::BrowserUseExternal.stage(), Stage::Stable);
+    assert_eq!(Feature::BrowserUseExternal.default_enabled(), true);
+    assert_eq!(
+        feature_for_key("browser_use_external"),
+        Some(Feature::BrowserUseExternal)
+    );
+
     assert_eq!(Feature::ComputerUse.stage(), Stage::Stable);
     assert_eq!(Feature::ComputerUse.default_enabled(), true);
     assert_eq!(feature_for_key("computer_use"), Some(Feature::ComputerUse));