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_themes.py

@@ -21,7 +21,9 @@ from app.api.deps import get_current_admin_api, get_db
 from app.services.vendor_theme_service import vendor_theme_service
 from models.database.user import User
 from models.schema.vendor_theme import (
+    ThemeDeleteResponse,
     ThemePresetListResponse,
+    ThemePresetResponse,
     VendorThemeResponse,
     VendorThemeUpdate,
 )
@@ -51,7 +53,7 @@ async def get_theme_presets(current_admin: User = Depends(get_current_admin_api)
     logger.info("Getting theme presets")
 
     presets = vendor_theme_service.get_available_presets()
-    return {"presets": presets}
+    return ThemePresetListResponse(presets=presets)
 
 
 # ============================================================================
@@ -134,7 +136,7 @@ async def update_vendor_theme(
     # Service handles all validation and raises appropriate exceptions
     # Global exception handler converts them to proper HTTP responses
     theme = vendor_theme_service.update_theme(db, vendor_code, theme_data)
-    return theme.to_dict()
+    return VendorThemeResponse(**theme.to_dict())
 
 
 # ============================================================================
@@ -142,7 +144,7 @@ async def update_vendor_theme(
 # ============================================================================
 
 
-@router.post("/{vendor_code}/preset/{preset_name}")
+@router.post("/{vendor_code}/preset/{preset_name}", response_model=ThemePresetResponse)
 async def apply_theme_preset(
     vendor_code: str = Path(..., description="Vendor code"),
     preset_name: str = Path(..., description="Preset name"),
@@ -184,10 +186,10 @@ async def apply_theme_preset(
     # Global exception handler converts to HTTP 404
     theme = vendor_theme_service.apply_theme_preset(db, vendor_code, preset_name)
 
-    return {
-        "message": f"Applied {preset_name} preset successfully",
-        "theme": theme.to_dict(),
-    }
+    return ThemePresetResponse(
+        message=f"Applied {preset_name} preset successfully",
+        theme=VendorThemeResponse(**theme.to_dict()),
+    )
 
 
 # ============================================================================
@@ -195,7 +197,7 @@ async def apply_theme_preset(
 # ============================================================================
 
 
-@router.delete("/{vendor_code}")
+@router.delete("/{vendor_code}", response_model=ThemeDeleteResponse)
 async def delete_vendor_theme(
     vendor_code: str = Path(..., description="Vendor code"),
     db: Session = Depends(get_db),
@@ -224,4 +226,4 @@ async def delete_vendor_theme(
     # Service handles deletion and raises exceptions if needed
     # Global exception handler converts them to proper HTTP responses
     result = vendor_theme_service.delete_theme(db, vendor_code)
-    return result
+    return ThemeDeleteResponse(message=result.get("message", "Theme deleted successfully"))