fix: update test fixtures for VendorTheme and CustomerPreferences schemas

- Update VendorTheme fixture to use new colors dict structure
- Update theme tests to check colors dict instead of individual fields
- Update CustomerPreferencesUpdate test for preferred_language field

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/middleware/test_theme_loading_flow.py

@@ -347,8 +347,14 @@ class TestThemeLoadingFlow:
 
         theme = VendorTheme(
             vendor_id=vendor_with_custom_domain.id,
-            primary_color="#123456",
-            secondary_color="#654321",
+            colors={
+                "primary": "#123456",
+                "secondary": "#654321",
+                "accent": "#ec4899",
+                "background": "#ffffff",
+                "text": "#1f2937",
+                "border": "#e5e7eb",
+            },
         )
         db.add(theme)
         db.commit()