fix(loyalty): read Google Wallet config from core settings instead of module config

Module config only reads from os.environ (not .env), so wallet settings
were always None. Core Settings already loads these via env_file=".env".
Also adds comprehensive wallet creation tests with mocked Google API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/loyalty/config.py

@@ -29,10 +29,6 @@ class ModuleConfig(BaseSettings):
     # Points configuration
     default_points_per_euro: int = 10  # 10 points per euro spent
 
-    # Google Wallet
-    google_issuer_id: str | None = None
-    google_service_account_json: str | None = None  # Path to JSON file
-
     # Apple Wallet
     apple_pass_type_id: str | None = None
     apple_team_id: str | None = None