fix: skip schema tests when jsonschema is absent

tests/test_optional_dependencies.py

@@ -0,0 +1,55 @@
+"""The suite runs with only the build dependencies installed.
+
+jsonschema is needed by the schema validator and by CI, nowhere else. Six
+tests imported it unguarded, so a contributor with stdlib and pyyaml saw
+fifteen errors instead of skipped schema checks.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+HIDE_AND_RUN = """
+import io
+import sys
+import unittest
+
+sys.modules["jsonschema"] = None
+names = sys.argv[1:]
+suite = unittest.defaultTestLoader.loadTestsFromNames(names)
+result = unittest.TextTestRunner(stream=io.StringIO(), verbosity=0).run(suite)
+for test, trace in result.errors + result.failures:
+    print(test.id())
+    print(trace.strip().splitlines()[-1])
+sys.exit(0 if result.wasSuccessful() else 1)
+"""
+
+
+class SuiteWithoutJsonschema(unittest.TestCase):
+    def test_every_module_skips_instead_of_erroring(self):
+        names = sorted(
+            f"tests.{path.stem}"
+            for path in (REPO_ROOT / "tests").glob("test_*.py")
+            if path.stem != Path(__file__).stem
+            and any(
+                token in path.read_text(encoding="utf-8")
+                for token in ("jsonschema", "validate_schemas")
+            )
+        )
+        completed = subprocess.run(
+            [sys.executable, "-c", HIDE_AND_RUN, *names],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+            timeout=600,
+        )
+        self.assertEqual(completed.returncode, 0, completed.stdout + completed.stderr[-2000:])
+
+
+if __name__ == "__main__":
+    unittest.main()