🌐 feat: disable external engine if not configured

[berry-13]

Summary

https://github.com/danny-avila/LibreChat/pull/2836#issuecomment-2125759171

before

no external engine configured

after

no external engine configured

Change Type

• [x] New feature (non-breaking change which adds functionality)

Checklist

• [x] My code adheres to this project's style guidelines
• [x] I have performed a self-review of my own code
• [x] I have commented in any complex areas of my code
• [x] I have made pertinent documentation changes
• [x] My changes do not introduce new warnings
• [x] I have written tests demonstrating that my changes are effective or that my feature works
• [x] Local unit tests pass with my changes
• [x] Any changes dependent on mine have been merged and published in downstream modules.
• [x] A pull request for updating the documentation has been submitted.