feat: implement MongoDB test database reset for clean test sessions

backend/tests/conftest.py

@@ -24,7 +24,8 @@ def set_test_db_env():
     os.environ['MONGO_URI'] = 'mongomock'
     os.environ['SECRET_KEY'] = TEST_SECRET_KEY
     os.environ['REFRESH_TOKEN_EXPIRY_DAYS'] = str(TEST_REFRESH_TOKEN_EXPIRY_DAYS)
-    # Ensure indexes are created once for the test session.  This is safe to
-    # call repeatedly because MongoDB treats index creation as idempotent.
-    from db.db import ensure_mongodb_indexes
+    # Drop the MongoDB test database once at the start of the session so every
+    # test begins from a clean slate.  Indexes are recreated afterward.
+    from db.db import reset_mongo_db, ensure_mongodb_indexes
+    reset_mongo_db()
     ensure_mongodb_indexes()