schemagen: drop support for additionalProperties: true

Following review feedback, the generator no longer supports permissive
objects. Changes:

- Reject `additionalProperties: true` at generation time.
- Treat an absent `additionalProperties` as `false`, so every object is
  strict by default and unknown keys are rejected during parsing.
- Remove the now-dead permissive-object infrastructure: `Kind::Skip`,
  `Cat::Skip`, `kSkip`, the per-frame `unknown` key set, and `isStrict()`.
- Update the README feature/rejection tables accordingly.
- Remove the permissive "loose" object from example.schema.json and the
  associated tests from test_gen.cpp.
- Add Python unit tests verifying the new `additionalProperties` behavior.

All tests pass (`ctest --output-on-failure`).

contrib/schemagen/test_schemagen.py

@@ -178,21 +178,69 @@ class SchemagenCollisionTest(unittest.TestCase):
         self.assertIn("Arr1", enumerators)
         self.assertEqual(len(enumerators), len(set(enumerators)))
 
-    def test_skip_is_reserved(self):
+    def test_skip_user_type_is_allowed(self):
         schema = {
             "type": "array",
             "items": {"$ref": "#/$defs/Skip"},
             "$defs": {"Skip": {"type": "object", "properties": {}}},
         }
         out = self.generate_and_compile(schema)
-        self.assertIn("struct Skip1", out)
+        self.assertIn("struct Skip", out)
+        self.assertNotIn("struct Skip1", out)
         m = re.search(r"enum class Kind : uint8_t \{([^}]+)\}", out)
         self.assertIsNotNone(m)
         enumerators = [e.strip() for e in m.group(1).split(",")]
         self.assertIn("Skip", enumerators)
-        self.assertIn("Skip1", enumerators)
+        self.assertIn("Arr0", enumerators)
         self.assertEqual(len(enumerators), len(set(enumerators)))
 
 
+class SchemagenAdditionalPropertiesTest(unittest.TestCase):
+    def run_schemagen(self, schema, args=None):
+        """Run schemagen on a schema dict. Returns (returncode, stdout, stderr)."""
+        with tempfile.NamedTemporaryFile("w", suffix=".json", delete=False) as fp:
+            json.dump(schema, fp)
+            schema_path = fp.name
+        try:
+            cmd = [sys.executable, SCRIPT, schema_path]
+            if args:
+                cmd.extend(args)
+            result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+            return result.returncode, result.stdout, result.stderr
+        finally:
+            os.unlink(schema_path)
+
+    def test_additional_properties_true_rejected(self):
+        schema = {
+            "type": "object",
+            "additionalProperties": True,
+            "properties": {"name": {"type": "string"}},
+        }
+        rc, stdout, stderr = self.run_schemagen(schema)
+        self.assertNotEqual(rc, 0)
+        self.assertIn("additionalProperties: true is not supported", stderr)
+
+    def test_additional_properties_absent_defaults_to_strict(self):
+        schema = {
+            "type": "object",
+            "properties": {"name": {"type": "string"}},
+        }
+        rc, stdout, stderr = self.run_schemagen(schema)
+        self.assertEqual(rc, 0, msg=stderr)
+        # The generated parser should reject unknown keys. Verify the key-matching
+        # helper returns -1 for an unknown key and cbKeyData rejects it.
+        self.assertIn("int matchKey(Kind k, std::string_view key) const {", stdout)
+        self.assertNotIn("bool isStrict(Kind k) const", stdout)
+
+    def test_additional_properties_false_accepted(self):
+        schema = {
+            "type": "object",
+            "additionalProperties": False,
+            "properties": {"name": {"type": "string"}},
+        }
+        rc, stdout, stderr = self.run_schemagen(schema)
+        self.assertEqual(rc, 0, msg=stderr)
+
+
 if __name__ == "__main__":
     unittest.main()