Add e2e tests for putFlattenReadOnlyModel, putFlattenUnknownModel, and withRelativeNextLink (#10361)

- [x] Add `testPageWithRelativeNextLink` test to `PageTests.java`
- [x] Add changelog entry for `@typespec/http-client-java`

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: weidongxu-microsoft <53292327+weidongxu-microsoft@users.noreply.github.com>
Co-authored-by: Weidong Xu <weidxu@microsoft.com>

Author: Copilot

.chronus/changes/add-test-withRelativeNextLink-2026-4-14.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: feature
+packages:
+  - "@typespec/http-client-java"
+---
+
+Add test case for `withRelativeNextLink`, `FlattenUnknownModel`, `FlattenReadOnlyModel` scenario.

packages/http-client-java/generator/http-client-generator-test/src/test/java/azure/clientgenerator/core/flattenproperty/FlattenClientTests.java

@@ -6,7 +6,11 @@
 import azure.clientgenerator.core.flattenproperty.models.ChildFlattenModel;
 import azure.clientgenerator.core.flattenproperty.models.ChildModel;
 import azure.clientgenerator.core.flattenproperty.models.FlattenModel;
+import azure.clientgenerator.core.flattenproperty.models.FlattenUnknownModel;
 import azure.clientgenerator.core.flattenproperty.models.NestedFlattenModel;
+import azure.clientgenerator.core.flattenproperty.models.Solution;
+import azure.clientgenerator.core.flattenproperty.models.SolutionProperties;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 public class FlattenClientTests {
@@ -19,4 +23,24 @@ public void testFlattenModel() {
         flattenClient.putNestedFlattenModel(
             new NestedFlattenModel("foo", new ChildFlattenModel("bar", new ChildModel("test", 10))));
     }
+
+    @Test
+    public void testPutFlattenUnknownModel() {
+        // flatten is ignored for unknown (non-model) type properties
+        FlattenUnknownModel result = flattenClient.putFlattenUnknownModel(new FlattenUnknownModel("foo"));
+        Assertions.assertEquals("test", result.getName());
+        Assertions.assertNotNull(result.getProperties());
+    }
+
+    @Test
+    public void testPutFlattenReadOnlyModel() {
+        // flatten with all read-only properties; input only contains writable fields
+        Solution result = flattenClient.putFlattenReadOnlyModel(new Solution("foo"));
+        Assertions.assertEquals("foo", result.getName());
+        SolutionProperties properties = result.getProperties();
+        Assertions.assertNotNull(properties);
+        Assertions.assertEquals("solution1", properties.getSolutionId());
+        Assertions.assertEquals("Solution Title", properties.getTitle());
+        Assertions.assertEquals("Solution Content", properties.getContent());
+    }
 }

packages/http-client-java/generator/http-client-generator-test/src/test/java/azure/core/page/PageTests.java

@@ -47,4 +47,9 @@ public void testPageRequestBody() {
     public void testPageNextLinkReInjectedParameters() {
         Assertions.assertEquals(2, client.withParameterizedNextLink("name", true).stream().count());
     }
+
+    @Test
+    public void testPageWithRelativeNextLink() {
+        Assertions.assertEquals(2, client.withRelativeNextLink().stream().count());
+    }
 }