Add Schema extension tests

Signed-off-by: Michael Edgar <michael@xlate.io>

Author: MikeEdgar

tck/src/main/java/org/eclipse/microprofile/openapi/tck/SchemaExtensionPropertyTest.java

@@ -0,0 +1,104 @@
+/**
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.eclipse.microprofile.openapi.tck;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.sameInstance;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import org.eclipse.microprofile.openapi.OASFactory;
+import org.eclipse.microprofile.openapi.models.media.Schema;
+import org.eclipse.microprofile.openapi.models.media.Schema.SchemaType;
+import org.testng.annotations.Test;
+
+/**
+ * This test covers the Extensible aspect of the Schema model, checking that properties are recognized correctly as
+ * extensions by an implementation and handled according to the Schema documentation.
+ */
+public class SchemaExtensionPropertyTest {
+
+    private static final String EXTNAME = "my-extension";
+
+    @Test
+    public void testExtensionSetForUnknownProperty() {
+        Schema schema = OASFactory.createSchema();
+        Object theExtension = new Object();
+        schema.set(EXTNAME, theExtension);
+
+        assertThat(schema.get(EXTNAME), is(sameInstance(theExtension)));
+        assertThat(schema.getExtension(EXTNAME), is(sameInstance(theExtension)));
+    }
+
+    @Test
+    public void testExtensionSetAllForUnknownProperty() {
+        Schema schema = OASFactory.createSchema();
+        Object theExtension = new Object();
+        schema.setAll(Map.of(
+                "type", List.of(SchemaType.STRING),
+                EXTNAME, theExtension));
+
+        assertThat(schema.getType(), is(List.of(SchemaType.STRING)));
+        assertThat(schema.get(EXTNAME), is(sameInstance(theExtension)));
+        assertThat(schema.getExtension(EXTNAME), is(sameInstance(theExtension)));
+    }
+
+    @Test
+    public void testExtensionAvailableFromGet() {
+        Schema schema = OASFactory.createSchema();
+        Object theExtension = new Object();
+        schema.addExtension(EXTNAME, theExtension);
+
+        assertThat(schema.get(EXTNAME), is(sameInstance(theExtension)));
+        assertThat(schema.getExtension(EXTNAME), is(sameInstance(theExtension)));
+    }
+
+    @Test
+    public void testExtensionSetWithNonnullDialect() {
+        Schema schema = OASFactory.createSchema();
+        schema.setSchemaDialect("https://spec.openapis.org/oas/3.1/dialect/base");
+        Object theExtension = new Object();
+        schema.set(EXTNAME, theExtension);
+        assertThat(schema.getExtension(EXTNAME), is(sameInstance(theExtension)));
+    }
+
+    @Test
+    public void testSetAllClearsExtensions() {
+        Schema schema = OASFactory.createSchema();
+        Object theExtension = new Object();
+        schema.set(EXTNAME, theExtension);
+        assertThat(schema.getExtension(EXTNAME), is(sameInstance(theExtension)));
+        schema.setAll(Collections.emptyMap());
+        assertThat(schema.hasExtension(EXTNAME), is(false));
+    }
+
+    @Test
+    public void testNullExtensionNotAdded() {
+        Schema schema = OASFactory.createSchema();
+
+        try {
+            schema.addExtension(EXTNAME, null);
+        } catch (Exception e) {
+            // May or may not be thrown
+        }
+
+        assertThat(schema.hasExtension(EXTNAME), is(false));
+    }
+}