Fix auth schemes: restore AccessTokenAuth (Bearer), add E2B_ACCESS_TOKEN support

- Keep AccessTokenAuth (type: http, scheme: bearer) for platform endpoints
- Remove /files from AUTH_EXEMPT_ENDPOINTS (only /health is exempt)
- Add E2B_ACCESS_TOKEN env var to validate script for Bearer auth testing
- Pass sandbox access token to /files endpoints in validate script
- Fix sandboxID server variable default to $SANDBOX_ID

Author: beran-t