Change email booking field's editable property from 'system' to
'system-but-optional' across all code paths. This allows email field
to be optional when phone field is required instead.
- Updated systemBeforeFieldEmail in API v2024_06_14 transformers
- Fixed explicit override in API v2024_04_15 service
- Updated test utility to match production behavior
- Ensures consistency with validation logic that requires either
email OR phone to be visible and required
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>