chore(ec2): add r8gn instance classes (#35536)

### Reason for this change
https://aws.amazon.com/about-aws/whats-new/2025/09/generally-available-amazon-ec2-r8gn-instances/

### Description of changes
Add r8gn instance classes

### Description of how you validated changes
```console
$ aws ec2 describe-instance-types | grep -e r8gn
            "InstanceType": "r8gn.12xlarge",
            "InstanceType": "r8gn.2xlarge",
...
```

### 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: phuhung273

packages/aws-cdk-lib/aws-ec2/lib/instance-types.ts

@@ -492,6 +492,16 @@ export enum InstanceClass {
    */
   R8GD = 'r8gd',
 
+  /**
+   * Memory optimized instances, 8th generation with Graviton4 processors and high network bandwidth capabilities
+   */
+  MEMORY8_GRAVITON4_HIGH_NETWORK_BANDWIDTH = 'memory8-graviton4-high-network-bandwidth',
+
+  /**
+   * Memory optimized instances, 8th generation with Graviton4 processors and high network bandwidth capabilities
+   */
+  R8GN = 'r8gn',
+
   /**
    * Compute optimized instances, 3rd generation
    */
@@ -1849,6 +1859,8 @@ export class InstanceType {
       [InstanceClass.R8G]: 'r8g',
       [InstanceClass.MEMORY8_GRAVITON4_NVME_DRIVE]: 'r8gd',
       [InstanceClass.R8GD]: 'r8gd',
+      [InstanceClass.MEMORY8_GRAVITON4_HIGH_NETWORK_BANDWIDTH]: 'r8gn',
+      [InstanceClass.R8GN]: 'r8gn',
       [InstanceClass.MEMORY8_INTEL_BASE]: 'r8i',
       [InstanceClass.R8I]: 'r8i',
       [InstanceClass.MEMORY8_INTEL_FLEX]: 'r8i-flex',