fix(plugins): hide bundled xai plugin from sync

[btc69m979y-dotcom]

Summary

• add the OpenClaw built-in xai provider plugin to the hidden plugin sync list
• add coverage for hiding xai from user plugin sync

Verification

• npm test -- pluginManager
• npx eslint --ext ts,tsx --report-unused-disable-directives --max-warnings 0 src/main/plugins/pluginManager.ts src/main/plugins/pluginManager.test.ts