feat: add typed DNS and DynDNS management for the KAS API

[emuuu]

Adds typed wrappers for the KAS API's DNS and DynDNS function groups, which previously
were only reachable through the generic RequestAsync escape hatch. Follows the existing
conventions (input models with ToParameters, Raw+FromMap read models, Y/N booleans,
scalar-id extraction).

DNS zone records (get/add/update/delete/reset_dns_settings)

• GetDnsRecordsAsync, GetDnsRecordAsync (record_id filter), AddDnsRecordAsync (returns the
  generated record_id), UpdateDnsRecordAsync, DeleteDnsRecordAsync, ResetDnsSettingsAsync
• DnsRecord read model, DnsRecordType enum + raw-type escape for unknown types
• Automatic zone_host trailing-dot normalization; empty record_name = zone apex
• ⚠️ ResetDnsSettingsAsync is destructive (discards all custom records) — flagged in docs/CHANGELOG/README
• DNS write actions require the account's dns_settings permission (else KAS fault dns_settings_not_allowed)

DynDNS users (add/get/update/delete_ddnsuser)

• AddDynDnsUserAsync (returns the generated dyndns_login), GetDynDnsUsersAsync,
  GetDynDnsUserAsync, UpdateDynDnsUserAsync, DeleteDynDnsUserAsync
• DynDnsUser read model incl. dyndns_target_ipv4/ipv6
• Documented KAS quirk: get_ddnsusers filters on ddns_login, but update/delete use dyndns_login

Verification

• All parameters verified against the official KAS phpdoc — nothing guessed. Corrected several
  assumptions (record_data not record_value; record_aux is a single field; update/get take no
  nameserver/record_type).
• Request/response shapes, add-return scalars and real response field names verified live against a
  real account (zone verqo.de), including a full write cycle (add→get→update→delete + cleanup).
  Confirmed: add_* returns record_id / dyndns_login as a string scalar; responses use record_zone
  (not zone_host) and dyndns_target_ipv4/ipv6; dual_stack is Y/N.

Tests / build

• 176 NUnit tests green (net10.0). Library compiles for net8.0/9.0/10.0 with 0 warnings.

Follow-up (separate PR)

• Transport seam: KasClient takes the concrete KasSoapTransport (no interface), so the action
  names/parameters the wrappers send aren't unit-testable. A dedicated refactor (IKasTransport +
  wrapper-level tests) is tracked and will come as its own PR.