fix: Ubuntu 24.04 等 PEP668 系统 Python 安装时自动加 --break-system-packages（安装/升级/卸载全链路）

tests/test_upgrade.py

@@ -230,3 +230,41 @@ class TestFetch:
         dest = tmp_path / "out.tar.gz"
         up_mod._download("http://x/f", "u", "p", dest)
         assert dest.read_bytes() == payload
+
+
+class TestPipInstallFlags:
+    def test_uses_break_system_packages_on_externally_managed(
+        self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        import sysconfig
+
+        em = tmp_path / "EXTERNALLY-MANAGED"
+        em.write_text("[externally-managed]\n")
+        monkeypatch.setattr(
+            sysconfig, "get_path", lambda name: str(tmp_path) if name == "stdlib" else ""
+        )
+        seen: list[list[str]] = []
+        monkeypatch.setattr(
+            "subprocess.run",
+            lambda cmd, **kw: seen.append(cmd)
+            or type("R", (), {"returncode": 0, "stderr": ""})(),
+        )
+        up_mod._pip_install(tmp_path / "pkg")
+        assert "--break-system-packages" in seen[0]
+
+    def test_skips_flag_without_externally_managed(
+        self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        import sysconfig
+
+        monkeypatch.setattr(
+            sysconfig, "get_path", lambda name: str(tmp_path) if name == "stdlib" else ""
+        )
+        seen: list[list[str]] = []
+        monkeypatch.setattr(
+            "subprocess.run",
+            lambda cmd, **kw: seen.append(cmd)
+            or type("R", (), {"returncode": 0, "stderr": ""})(),
+        )
+        up_mod._pip_install(tmp_path / "pkg")
+        assert "--break-system-packages" not in seen[0]