fix(api): use proper Pydantic models in vendor themes/domains

vendor_themes.py:
- Return ThemePresetListResponse instead of raw dict (API-001 fix)
- Add ThemePresetResponse response_model to apply_theme_preset
- Add ThemeDeleteResponse response_model to delete endpoint

vendor_domain.py:
- Remove _get_vendor_by_id helper with direct DB query
- Use vendor_service.get_vendor_by_id() instead (API-002 fix)

models/schema/vendor_theme.py:
- Add ThemeDeleteResponse model for delete endpoint response

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/api/v1/admin/vendor_domains.py

@@ -5,7 +5,7 @@ Admin endpoints for managing vendor custom domains.
 Follows the architecture pattern:
 - Endpoints only handle HTTP layer
 - Business logic in service layer
-- Proper exception handling
+- Domain exceptions bubble up to global handler
 - Pydantic schemas for validation
 """
 
@@ -16,10 +16,9 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api
 from app.core.database import get_db
-from app.exceptions import VendorNotFoundException
 from app.services.vendor_domain_service import vendor_domain_service
+from app.services.vendor_service import vendor_service
 from models.database.user import User
-from models.database.vendor import Vendor
 from models.schema.vendor_domain import (
     DomainDeletionResponse,
     DomainVerificationInstructions,
@@ -34,26 +33,6 @@ router = APIRouter(prefix="/vendors")
 logger = logging.getLogger(__name__)
 
 
-def _get_vendor_by_id(db: Session, vendor_id: int) -> Vendor:
-    """
-    Helper to get vendor by ID.
-
-    Args:
-        db: Database session
-        vendor_id: Vendor ID
-
-    Returns:
-        Vendor object
-
-    Raises:
-        VendorNotFoundException: If vendor not found
-    """
-    vendor = db.query(Vendor).filter(Vendor.id == vendor_id).first()
-    if not vendor:
-        raise VendorNotFoundException(str(vendor_id), identifier_type="id")
-    return vendor
-
-
 @router.post("/{vendor_id}/domains", response_model=VendorDomainResponse)
 def add_vendor_domain(
     vendor_id: int = Path(..., description="Vendor ID", gt=0),
@@ -122,8 +101,8 @@ def list_vendor_domains(
     **Raises:**
     - 404: Vendor not found
     """
-    # Verify vendor exists
-    _get_vendor_by_id(db, vendor_id)
+    # Verify vendor exists (raises VendorNotFoundException if not found)
+    vendor_service.get_vendor_by_id(db, vendor_id)
 
     domains = vendor_domain_service.get_vendor_domains(db, vendor_id)