refactor(dns): extend DnsValidator to DnsManager with DNS record creation

- 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

ts/mail/routing/index.ts

@@ -1,6 +1,6 @@
 // Email routing components
 export * from './classes.email.router.js';
 export * from './classes.unified.email.server.js';
-export * from './classes.dnsmanager.js';
+export * from './classes.dns.manager.js';
 export * from './interfaces.js';
 export * from './classes.domain.registry.js';