[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/core/config.schema.json

@@ -373,6 +373,9 @@
             "browser_use": {
               "type": "boolean"
             },
+            "browser_use_external": {
+              "type": "boolean"
+            },
             "child_agents_md": {
               "type": "boolean"
             },
@@ -3390,6 +3393,9 @@
         "browser_use": {
           "type": "boolean"
         },
+        "browser_use_external": {
+          "type": "boolean"
+        },
         "child_agents_md": {
           "type": "boolean"
         },