Add Domain (tenant) CRUD to Client SDK v2

Implement v2 client for Domain (tenant) management using DTOs defined in BA-4543.

### V2 Client (client/v2/domains/domain.py)
- DomainClient(BaseDomainClient) with typed_request() calls:
- create(request) -> POST /admin/domains
- get(domain_name) -> GET /admin/domains/{domain_name}
- search(request) -> POST /admin/domains/search
- update(domain_name, request) -> PATCH /admin/domains/{domain_name}
- delete(request) -> POST /admin/domains/delete
- purge(request) -> POST /admin/domains/purge
- Register domain property in BackendAIClientRegistry

### DTO
Reuse DTOs from common/dto/manager/domain/ (defined in BA-4543).

### Reference
- Handler: BA-4543 (must be completed first)
- V1 client: client/func/domain.py

JIRA Issue: BA-4532