The DnsManager was still checking for the old dnsDomain property that was
replaced by dnsNsDomains and dnsScopes in the DNS Architecture Improvements.
Changes:
- Replace dnsDomain checks with dnsNsDomains and dnsScopes validation
- Add check to ensure email domain is included in dnsScopes array
- Update NS delegation check to work with multiple nameservers
- Update error messages to guide users to the new configuration format
- Rename DnsValidator to DnsManager to better reflect its expanded responsibilities
- Move DNS record creation logic from UnifiedEmailServer to DnsManager
- Add ensureDnsRecords() method that handles both validation and creation
- Consolidate internal DNS record creation (MX, SPF, DMARC) in one place
- Keep DKIM key generation in UnifiedEmailServer but move DNS registration to DnsManager
- Update all imports and tests to use DnsManager instead of DnsValidator
- Improve code organization and discoverability of DNS functionality
