Expose managed policy for browser settings imports (#39720)

## What changed

- Add `in_app_browser.allow_external_browser_settings_import` to managed requirements, preserving explicit Boolean values through layered composition while leaving an omitted value unset.
- Return the policy as `inAppBrowser.allowExternalBrowserSettingsImport` from `configRequirements/read` and include it in the generated protocol schemas.
- Keep the import policy independent from the in-app browser feature flag and agent Browser Use requirements.

## Testing

- Cover parsing, managed-layer precedence, invalid values, user and session override resistance, and app-server response serialization.

GitOrigin-RevId: efa2621d2b1cf503f1bee2505d9914cb4fb7221d

codex-rs/core/src/config/config_tests.rs

@@ -9844,6 +9844,7 @@ async fn test_requirements_web_search_mode_allowlist_does_not_warn_when_unset()
         allow_remote_control: None,
         computer_use: None,
         browser_use: None,
+        in_app_browser: None,
         windows: None,
         feature_requirements: None,
         hooks: None,