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

test/test.dns-mode-switching.ts

@@ -2,7 +2,7 @@ import { tap, expect } from '@git.zone/tstest/tapbundle';
 import * as plugins from '../ts/plugins.js';
 import * as paths from '../ts/paths.js';
 import { StorageManager } from '../ts/storage/classes.storagemanager.js';
-import { DnsValidator } from '../ts/mail/routing/classes.dns.validator.js';
+import { DnsManager } from '../ts/mail/routing/classes.dns.manager.js';
 import { DomainRegistry } from '../ts/mail/routing/classes.domain.registry.js';
 import { DKIMCreator } from '../ts/mail/security/classes.dkimcreator.js';
 import type { IEmailDomainConfig } from '../ts/mail/routing/interfaces.js';