fix(rds): DatabaseCluster.instanceEndpoints doesn't include writer endpoint (#29337)

### Issue # (if applicable)

Closes #29279.

### Reason for this change

`DatabaseCluster.instanceEndpoints` should include writer's endpoint but doesn't.

### Description of changes

Add writer's endpoint to `DatabaseCluster.instanceEndpoints`

### Description of how you validated changes

Add unit tests

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: badmintoncryer

packages/aws-cdk-lib/aws-rds/lib/cluster.ts

@@ -756,6 +756,7 @@ abstract class DatabaseClusterNew extends DatabaseClusterBase {
       promotionTier: 0, // override the promotion tier so that writers are always 0
     });
     instanceIdentifiers.push(writer.instanceIdentifier);
+    instanceEndpoints.push(new Endpoint(writer.dbInstanceEndpointAddress, this.clusterEndpoint.port));
 
     (props.readers ?? []).forEach(instance => {
       const clusterInstance = instance.bind(this, this, {

packages/aws-cdk-lib/aws-rds/test/cluster.test.ts

@@ -528,6 +528,103 @@ describe('cluster new api', () => {
     });
   });
 
+  describe('instanceEndpoints', () => {
+    test('should contain writer and reader instance endpoints at DatabaseCluster', () => {
+      //GIVEN
+      const stack = testStack();
+      const vpc = new ec2.Vpc(stack, 'VPC');
+
+      //WHEN
+      const cluster = new DatabaseCluster(stack, 'Database', {
+        engine: DatabaseClusterEngine.AURORA,
+        vpc,
+        writer: ClusterInstance.serverlessV2('writer'),
+        readers: [ClusterInstance.serverlessV2('reader')],
+        iamAuthentication: true,
+      });
+
+      //THEN
+      expect(cluster.instanceEndpoints).toHaveLength(2);
+      expect(stack.resolve(cluster.instanceEndpoints)).toEqual([{
+        hostname: {
+          'Fn::GetAtt': ['Databasewriter2462CC03', 'Endpoint.Address'],
+        },
+        port: {
+          'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'],
+        },
+        socketAddress: {
+          'Fn::Join': ['', [
+            { 'Fn::GetAtt': ['Databasewriter2462CC03', 'Endpoint.Address'] },
+            ':',
+            { 'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'] },
+          ]],
+        },
+      }, {
+        hostname: {
+          'Fn::GetAtt': ['Databasereader13B43287', 'Endpoint.Address'],
+        },
+        port: {
+          'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'],
+        },
+        socketAddress: {
+          'Fn::Join': ['', [
+            { 'Fn::GetAtt': ['Databasereader13B43287', 'Endpoint.Address'] },
+            ':',
+            { 'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'] },
+          ]],
+        },
+      }]);
+    });
+
+    test('should contain writer and reader instance endpoints at DatabaseClusterFromSnapshot', () => {
+      //GIVEN
+      const stack = testStack();
+      const vpc = new ec2.Vpc(stack, 'VPC');
+
+      //WHEN
+      const cluster = new DatabaseClusterFromSnapshot(stack, 'Database', {
+        engine: DatabaseClusterEngine.AURORA,
+        vpc,
+        snapshotIdentifier: 'snapshot-identifier',
+        iamAuthentication: true,
+        writer: ClusterInstance.serverlessV2('writer'),
+        readers: [ClusterInstance.serverlessV2('reader')],
+      });
+
+      //THEN
+      expect(cluster.instanceEndpoints).toHaveLength(2);
+      expect(stack.resolve(cluster.instanceEndpoints)).toEqual([{
+        hostname: {
+          'Fn::GetAtt': ['Databasewriter2462CC03', 'Endpoint.Address'],
+        },
+        port: {
+          'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'],
+        },
+        socketAddress: {
+          'Fn::Join': ['', [
+            { 'Fn::GetAtt': ['Databasewriter2462CC03', 'Endpoint.Address'] },
+            ':',
+            { 'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'] },
+          ]],
+        },
+      }, {
+        hostname: {
+          'Fn::GetAtt': ['Databasereader13B43287', 'Endpoint.Address'],
+        },
+        port: {
+          'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'],
+        },
+        socketAddress: {
+          'Fn::Join': ['', [
+            { 'Fn::GetAtt': ['Databasereader13B43287', 'Endpoint.Address'] },
+            ':',
+            { 'Fn::GetAtt': ['DatabaseB269D8BB', 'Endpoint.Port'] },
+          ]],
+        },
+      }]);
+    });
+  });
+
   describe('provisioned writer with serverless readers', () => {
     test('serverless reader in promotion tier 2 throws warning', () => {
       // GIVEN