fix(测试): 清理本次创建的隔离资源并保留失败码

记录数据库与账号的创建结果，只清理本次成功创建的对象。
支持显式保留资源，拒绝误删复用库或重名对象，补充退出码回归。

backend/tests/unit/test_isolated_runner.py

@@ -0,0 +1,88 @@
+"""运行器必须保留失败码，并仅清理本次成功创建的对象。"""
+
+import importlib.util
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+
+@pytest.fixture
+def runner(monkeypatch):
+    path = Path(__file__).resolve().parents[3] / "scripts/run_isolated_tests.py"
+    spec = importlib.util.spec_from_file_location("isolated_runner", path)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    calls, commands = [], []
+
+    def sql(statement):
+        calls.append(statement)
+        return SimpleNamespace(returncode=0)
+
+    def run(command, **kwargs):
+        commands.append(command)
+        return SimpleNamespace(returncode=0)
+
+    monkeypatch.setattr(module, "run_mysql", sql)
+    monkeypatch.setattr(module.subprocess, "run", run)
+    monkeypatch.setattr(module.sys, "argv", ["run_isolated_tests.py"])
+    return module, calls, commands
+
+
+@pytest.mark.parametrize(
+    "args", [[], ["--keep"], ["--reuse-database", "payroll_test_0123456789ab"]]
+)
+def test_失败码不被清理覆盖(runner, monkeypatch, args):
+    module, calls, _ = runner
+    monkeypatch.setattr(module.sys, "argv", ["runner", *args])
+    monkeypatch.setattr(module.subprocess, "run", lambda *_, **__: SimpleNamespace(returncode=5))
+    assert module.main() == 5
+    drops = [sql for sql in calls if sql.startswith("DROP")]
+    assert bool(drops) == ("--keep" not in args)
+    if "--reuse-database" in args:
+        assert not any(sql.startswith("DROP DATABASE") for sql in drops)
+
+
+def test_只清理成功创建的对象(runner, monkeypatch):
+    module, calls, _ = runner
+
+    def sql(statement):
+        calls.append(statement)
+        return SimpleNamespace(returncode=int(statement.startswith("CREATE USER")))
+
+    monkeypatch.setattr(module, "run_mysql", sql)
+    assert module.main() == 1
+    assert any(sql.startswith("DROP DATABASE") for sql in calls)
+    assert not any(sql.startswith("DROP USER") for sql in calls)
+
+
+def test_创建重名库失败绝不删除原库(runner, monkeypatch):
+    module, calls, _ = runner
+
+    def sql(statement):
+        calls.append(statement)
+        return SimpleNamespace(returncode=1)
+
+    monkeypatch.setattr(module, "run_mysql", sql)
+    assert module.main() == 1
+    assert not any(sql.startswith("DROP") for sql in calls)
+
+
+def test_参数分隔符不传给pytest(runner, monkeypatch):
+    module, _, commands = runner
+    monkeypatch.setattr(module.sys, "argv", ["runner", "--", "tests/integration/test_auth_flow.py"])
+    assert module.main() == 0
+    assert "--" not in commands[-1]
+    assert "tests/integration/test_auth_flow.py" in commands[-1]
+
+
+def test_清理失败不能报成功(runner, monkeypatch):
+    module, calls, _ = runner
+
+    def sql(statement):
+        calls.append(statement)
+        return SimpleNamespace(returncode=int(statement.startswith("DROP DATABASE")))
+
+    monkeypatch.setattr(module, "run_mysql", sql)
+    assert module.main() == 1
+    assert any(sql.startswith("DROP USER") for sql in calls)