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

---------

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

tests/test_main_direct_execution.py

@@ -14,8 +14,7 @@ def test_main_script_does_not_shadow_stdlib_platform(tmp_path):
         (
             "import runpy",
             "import sys",
-            # CI 无 app.application.site.sites 二进制模块，先补垫片再执行 main.py（与 conftest 同源）；
-            # 必须在篡改 sys.path / 摘除 platform 之前调用，避免真实依赖链受探针环境影响。
+            # 独立探针不经过 pytest 引导，先隔离站点原生制品，再改变模块搜索路径。
             "from app.testing.bootstrap import ensure_sites_stub",
             "ensure_sites_stub()",
             f"sys.path.insert(0, {str(MAIN_PATH.parent)!r})",