refactor: introduce IKasTransport seam and add wrapper-level client tests

[emuuu]

Introduces a transport seam so the action names and parameters the typed wrappers send are finally
unit-testable. Internal-only refactor — no public API change and no transport behaviour change.

Seam

• New internal IKasTransport (Soap/IKasTransport.cs) with the existing CallAsync(action, params, ct) signature
• KasSoapTransport implements it; KasClient now depends on IKasTransport
• DI aliases IKasTransport onto the same KasSoapTransport singleton (no second transport), so the
  shared session token and flood window are preserved

Tests (+52)

• RecordingKasTransport fake (throws on empty queue → catches forgotten enqueues / extra calls)
• Contract tests per wrapper group (Account/Mail/DNS/DynDNS) asserting the exact KAS action name, the
  parameter dictionary, and the response mapping — incl. KAS quirks: zone_host trailing-dot, apex
  record_name, record_aux, ddns_login vs dyndns_login, Y/N + tri-state, scalar-id extraction
• Seam tests: RequestAsync passthrough (incl. token), DI single-instance invariant

Review

• Passed code review: 0 production findings, 4 test findings (all low/medium) — all applied.

Build / tests

• net8.0/9.0/10.0: 0 warnings, 0 errors. 227 tests green (net10.0). Working tree clean.