fix(install): provision minitest next to ruby on Arch and the RedHat family

The Manjaro workspace job failed test-unit-ruby with "cannot load such
file -- minitest". Debian vendors minitest in libruby, but Arch ships it
as ruby-minitest and Fedora and CentOS Stream as rubygem-minitest, none
of which ruby pulls in. ensure_ruby_present now probes
`require "minitest"` and installs the distro package when the probe
fails, then fails loudly if minitest is still missing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/unit/python/utils/install/test_ruby.py

@@ -9,19 +9,75 @@ from utils.install import ruby as ruby_mod
 
 
 class TestEnsureRubyPresent(unittest.TestCase):
-    def test_delegates_to_the_shared_provisioner(self) -> None:
-        with mock.patch.object(ruby_mod, "ensure_command_present") as ensure:
+    def test_provisions_ruby_then_minitest(self) -> None:
+        calls = mock.Mock()
+        with (
+            mock.patch.object(ruby_mod, "ensure_command_present", calls.ruby),
+            mock.patch.object(ruby_mod, "ensure_minitest_present", calls.minitest),
+        ):
             ruby_mod.ensure_ruby_present()
-        ensure.assert_called_once_with("ruby")
+        self.assertEqual(
+            calls.mock_calls, [mock.call.ruby("ruby"), mock.call.minitest()]
+        )
 
     def test_propagates_the_hard_failure(self) -> None:
         with (
             mock.patch.object(
                 ruby_mod, "ensure_command_present", side_effect=RuntimeError("boom")
             ),
+            mock.patch.object(ruby_mod, "ensure_minitest_present") as minitest,
             self.assertRaises(RuntimeError),
         ):
             ruby_mod.ensure_ruby_present()
+        minitest.assert_not_called()
+
+
+class TestEnsureMinitestPresent(unittest.TestCase):
+    def run_with(self, *, loadable: list[bool], manager: str) -> mock.Mock:
+        """Run ``ensure_minitest_present`` with stubbed probe and installer.
+
+        Args:
+            loadable: successive results of the minitest probe.
+            manager: package manager the host reports.
+
+        Returns:
+            The stubbed ``install_package_candidates``.
+        """
+        with (
+            mock.patch.object(ruby_mod, "minitest_loadable", side_effect=loadable),
+            mock.patch.object(ruby_mod, "detect_package_manager", return_value=manager),
+            mock.patch.object(ruby_mod, "install_package_candidates") as install,
+            mock.patch.object(ruby_mod, "log"),
+        ):
+            ruby_mod.ensure_minitest_present()
+        return install
+
+    def test_a_loadable_minitest_installs_nothing(self) -> None:
+        install = self.run_with(loadable=[True], manager="pacman")
+        install.assert_not_called()
+
+    def test_arch_installs_ruby_minitest(self) -> None:
+        install = self.run_with(loadable=[False, True], manager="pacman")
+        install.assert_called_once_with("pacman", ["ruby-minitest"])
+
+    def test_the_redhat_family_installs_rubygem_minitest(self) -> None:
+        install = self.run_with(loadable=[False, True], manager="dnf")
+        install.assert_called_once_with("dnf", ["rubygem-minitest"])
+
+    def test_still_missing_after_the_install_fails_loudly(self) -> None:
+        with self.assertRaises(RuntimeError):
+            self.run_with(loadable=[False, False], manager="pacman")
+
+    def test_an_unmapped_manager_fails_without_installing(self) -> None:
+        with (
+            mock.patch.object(ruby_mod, "minitest_loadable", return_value=False),
+            mock.patch.object(ruby_mod, "detect_package_manager", return_value="brew"),
+            mock.patch.object(ruby_mod, "install_package_candidates") as install,
+            mock.patch.object(ruby_mod, "log"),
+            self.assertRaises(RuntimeError),
+        ):
+            ruby_mod.ensure_minitest_present()
+        install.assert_not_called()
 
 
 if __name__ == "__main__":