fix: say how to run the schema check step 0 skips

Without jsonschema the step printed only the missing module, and the run then read COMPLETE while PR validation runs the same check. The skip now names the install command and the PR check.

tests/test_tool_messages.py

@@ -181,6 +181,27 @@ class ThePipelineCountsItsRealSteps(unittest.TestCase):
         self.assertEqual(title, f"8b render site [{total}/{total}]")
         self.assertNotIn("/8", pipeline.step_title(pipeline.STEP_LABELS["verify"]))
 
+    def test_a_skipped_schema_check_says_how_to_run_it(self):
+        """Step 0 said only "jsonschema not installed", then the run read
+        COMPLETE and the schema failed in PR validation."""
+        argv = ["pipeline.py", "--offline", "--skip-packs", "--skip-docs"]
+        out = io.StringIO()
+        real_find_spec = pipeline.importlib.util.find_spec
+        with mock.patch.object(sys, "argv", argv), mock.patch.object(
+            pipeline, "run", return_value=(True, "")
+        ), mock.patch.object(
+            pipeline.importlib.util, "find_spec",
+            side_effect=lambda name, *a: None if name == "jsonschema"
+            else real_find_spec(name, *a),
+        ), contextlib.redirect_stdout(out), contextlib.suppress(SystemExit):
+            pipeline.main()
+        line = next(
+            row for row in out.getvalue().splitlines()
+            if "0 validate schemas" in row and "SKIPPED" in row
+        )
+        self.assertIn('pip install "jsonschema[format-nongpl]"', line)
+        self.assertIn("PR validation runs this check", line)
+
     def test_an_offline_buildbot_check_has_a_summary_row(self):
         argv = ["pipeline.py", "--offline", "--check-buildbot", "--skip-packs",
                 "--skip-docs"]