test: 隔离单测中的站点原生资源 (#6452)

* test: isolate native site resources

* refactor(test): retain sites stub helper contract

* fix(test): preserve sites stub probe imports

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Co-authored-by: jxxghp <jxxghp@gmail.com>

tests/test_database_index_migration.py

@@ -71,7 +71,7 @@ IDENTITY_INDEX_SIGNATURES = {
 CURRENT_SCHEMA_CHAIN_SCRIPT = """
 from app.testing.bootstrap import ensure_sites_stub
 
-# Alembic 会导入引用业务链的旧 revision；全新 CI 环境没有动态下发的 sites 模块。
+# Alembic 会导入引用业务链的旧 revision，迁移验证不应加载本机站点原生制品。
 ensure_sites_stub()
 
 from alembic.config import Config