feat(opensearchservice): support OI2 instance type with local NVMe storage (#36700)

### Issue # (if applicable)

Closes #36698

### Reason for this change

OI2 is an OpenSearch-specific instance type that uses local NVMe storage and does not support EBS volumes. Currently, CDK validation incorrectly rejects OI2 instance types when ebs: { enabled: false } is specified, throwing:
ValidationError: EBS volumes are required when using instance types other than R3, I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD or R8GD.


### Description of changes

- Added 'oi2' to the unSupportEbsInstanceType array in domain.ts to recognize OI2 as an instance type that doesn't support EBS (and therefore supports instance storage)
- Since OI2 is an OpenSearch-specific instance family (not an EC2 instance class), it's added as a string literal rather than using ec2.InstanceClass
- Updated the comment to include OI2 and added a reference to the AWS documentation
- Updated unit tests to include OI2 test cases and updated expected error messages

### Describe any new or updated permissions being added

N/A - No IAM permission changes.

### Description of how you validated changes

- Added unit tests for OI2 instance type:
  - Error when OI2 is specified with EBS enabled
  - No error when OI2 is specified without EBS enabled
- All existing tests continue to pass

### 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: pahud

packages/aws-cdk-lib/aws-opensearchservice/lib/domain.ts

@@ -1695,6 +1695,7 @@ export class Domain extends DomainBase implements IDomain, ec2.IConnectable {
       ec2.InstanceClass.IM4GN,
       ec2.InstanceClass.R7GD,
       ec2.InstanceClass.R8GD,
+      'oi2', // OpenSearch-specific instance type with local NVMe storage
     ];
 
     const supportInstanceStorageInstanceType = [
@@ -1731,8 +1732,9 @@ export class Domain extends DomainBase implements IDomain, ec2.IConnectable {
       throw new ValidationError(`${formatInstanceTypesList(unSupportUltraWarmInstanceType, 'and')} instance types do not support UltraWarm storage.`, this);
     }
 
-    // Only R3, I3, R6GD, I4G, I4I, IM4GN and R7GD support instance storage, per
+    // Only R3, I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD, R8GD and OI2 support instance storage, per
     // https://aws.amazon.com/opensearch-service/pricing/
+    // https://docs.aws.amazon.com/opensearch-service/latest/developerguide/supported-instance-types.html
     if (!ebsEnabled && !isEveryDatanodeInstanceType(...supportInstanceStorageInstanceType)) {
       throw new ValidationError(`EBS volumes are required when using instance types other than ${formatInstanceTypesList(supportInstanceStorageInstanceType, 'or')}.`, this);
     }

packages/aws-cdk-lib/aws-opensearchservice/test/domain.test.ts

@@ -2081,6 +2081,7 @@ each(testedOpenSearchVersions).describe('custom error responses', (engineVersion
     'i8g.4xlarge.search',
     'r7gd.xlarge.search',
     'r8gd.medium.search',
+    'oi2.large.search',
   ])('error when %s instance type is specified with EBS enabled', (dataNodeInstanceType) => {
     expect(() => new Domain(stack, 'Domain2', {
       version: engineVersion,
@@ -2091,7 +2092,7 @@ each(testedOpenSearchVersions).describe('custom error responses', (engineVersion
         volumeSize: 100,
         volumeType: EbsDeviceVolumeType.GENERAL_PURPOSE_SSD,
       },
-    })).toThrow(/I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD and R8GD instance types do not support EBS storage volumes./);
+    })).toThrow(/I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD, R8GD and OI2 instance types do not support EBS storage volumes./);
   });
 
   test.each([
@@ -2103,6 +2104,7 @@ each(testedOpenSearchVersions).describe('custom error responses', (engineVersion
     'i8g.4xlarge.search',
     'r7gd.xlarge.search',
     'r8gd.medium.search',
+    'oi2.large.search',
   ])('should not throw when %s instance type is specified without EBS enabled', (dataNodeInstanceType) => {
     expect(() => new Domain(stack, 'Domain2', {
       version: engineVersion,
@@ -2142,7 +2144,7 @@ each(testedOpenSearchVersions).describe('custom error responses', (engineVersion
   test.each([
     'm5.large.search',
     'r5.large.search',
-  ])('error when any instance type other than R3, I3, R6GD, I4I, I4G, IM4GN or R7GD are specified without EBS enabled', (masterNodeInstanceType) => {
+  ])('error when any instance type other than R3, I3, R6GD, I4I, I4G, I8G, IM4GN, R7GD, R8GD or OI2 are specified without EBS enabled', (masterNodeInstanceType) => {
     expect(() => new Domain(stack, 'Domain1', {
       version: engineVersion,
       ebs: {
@@ -2151,7 +2153,7 @@ each(testedOpenSearchVersions).describe('custom error responses', (engineVersion
       capacity: {
         masterNodeInstanceType,
       },
-    })).toThrow(/EBS volumes are required when using instance types other than R3, I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD or R8GD./);
+    })).toThrow(/EBS volumes are required when using instance types other than R3, I3, R6GD, I4G, I4I, I8G, IM4GN, R7GD, R8GD or OI2./);
   });
 
   test('can use compatible master instance types that does not have local storage when data node type is i3 or r6gd', () => {