test(upgrade): make pip-flag tests hermetic, cover uv branch

uv run injects VIRTUAL_ENV, which silently selects the uv pip branch and
broke test_uses_break_system_packages_on_externally_managed. Both tests
now delete VIRTUAL_ENV to pin the plain-pip branch; new test pins the
uv branch via mocks.

tests/test_upgrade.py

@@ -238,6 +238,9 @@ class TestPipInstallFlags:
     ) -> None:
         import sysconfig
 
+        # Force the plain-pip branch regardless of how pytest was launched
+        # (e.g. `uv run` sets VIRTUAL_ENV, which would select the uv branch).
+        monkeypatch.delenv("VIRTUAL_ENV", raising=False)
         em = tmp_path / "EXTERNALLY-MANAGED"
         em.write_text("[externally-managed]\n")
         monkeypatch.setattr(
@@ -257,6 +260,7 @@ class TestPipInstallFlags:
     ) -> None:
         import sysconfig
 
+        monkeypatch.delenv("VIRTUAL_ENV", raising=False)
         monkeypatch.setattr(
             sysconfig, "get_path", lambda name: str(tmp_path) if name == "stdlib" else ""
         )
@@ -268,3 +272,18 @@ class TestPipInstallFlags:
         )
         up_mod._pip_install(tmp_path / "pkg")
         assert "--break-system-packages" not in seen[0]
+
+    def test_prefers_uv_pip_inside_virtualenv(
+        self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setenv("VIRTUAL_ENV", str(tmp_path / "venv"))
+        monkeypatch.setattr(up_mod.shutil, "which", lambda name: f"/usr/bin/{name}")
+        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 seen[0][:3] == ["uv", "pip", "install"]
+        assert "--break-system-packages" not in seen[0]