fix(folder trust): add validation for trusted folder level

[adamfweidman]

Summary

This PR introduces validation for the trustLevel values in the trusted-folders.json configuration file. It ensures that only recognized trust levels are processed, preventing potential errors from invalid or misspelled entries.

Details

Previously GC started up with any string as a trustLevel without validation. This could lead to overriding the settings or ignoring them all together.

This change adds a type guard (isTrustLevel) to validate each entry when loading the trusted folders configuration.

Related Issues

Fixes 921

How to Validate

Add to the following to settings.json

security: {
        folderTrust: {
          enabled: true,
        },
      },

Add to .gemini/trustedFolders.json

{
  "<cwd>": "DO_NOT_TRUSTtest"
}

On main this will not cause an error, on this new branch it will prevent correctly prevent startup.

Pre-Merge Checklist

• [x] Added/updated tests (if needed)
• [ ] Validated on required platforms/methods:
  • [ ] MacOS
    • [x] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [ ] Linux
    • [ ] npm run
    • [ ] npx
    • [ ] Docker