feat(batch): add AL2023 image types and default to AL2023 (under feature flag)

[aws-joelherd]

Add missing enum values for AWS Batch Ec2Configuration:

• EcsMachineImageType.ECS_AL2023_NVIDIA
• EksMachineImageType.EKS_AL2023
• EksMachineImageType.EKS_AL2023_NVIDIA

Introduce feature flag @aws-cdk/aws-batch:defaultToAL2023 that switches the default imageType from AL2 to AL2023 for both ECS and EKS compute environments. Amazon Linux 2 has reached EOL for EKS (Nov 2025) and reaches EOL for ECS in June 2026.

For EKS compute environments with a launch template, automatically set userdataType to EKS_NODEADM when an AL2023 image type is resolved, as required by the AWS Batch API to ensure nodes join the cluster.

Update A1 instance validation to also reject ECS_AL2023_NVIDIA and to correctly resolve the feature-flag-defaulted imageType.

Issue # (if applicable)

Closes #35839, closes #37205.

Reason for this change

Amazon Linux 2 has reached EOL for EKS (Nov 2025) and reaches EOL for ECS
in June 2026. The AWS Batch service already defaults to AL2023 for newly
created compute environments, but CDK still hardcodes ECS_AL2 / EKS_AL2
when users don't specify an imageType.

Additionally, the ECS_AL2023_NVIDIA, EKS_AL2023, and EKS_AL2023_NVIDIA
image types supported by the Batch API were missing from the CDK enum definitions.

Related public documentation:

• https://docs.aws.amazon.com/batch/latest/APIReference/API_Ec2Configuration.html
• https://docs.aws.amazon.com/batch/latest/userguide/eks-al2-ami-deprecation.html
• https://docs.aws.amazon.com/batch/latest/userguide/eks-migration-2023.html
• https://docs.aws.amazon.com/batch/latest/userguide/ecs-al2-ami-deprecation.html
• https://docs.aws.amazon.com/batch/latest/userguide/ecs-migration-2023.html
• https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_batch.EcsMachineImageType.html
• https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_batch.EksMachineImageType.html

Description of changes

• Added EcsMachineImageType.ECS_AL2023_NVIDIA, EksMachineImageType.EKS_AL2023,
  and EksMachineImageType.EKS_AL2023_NVIDIA
• Introduced feature flag @aws-cdk/aws-batch:defaultToAL2023 that switches the
  default imageType from AL2 to AL2023 for both ECS and EKS compute environments
• For EKS compute environments with a launch template, automatically sets
  userdataType to EKS_NODEADM when an AL2023 image type is resolved, as
  required by the AWS Batch API to ensure nodes join the cluster
• Updated A1 instance validation to also reject ECS_AL2023_NVIDIA and to
  correctly resolve the feature-flag-defaulted imageType

Describe any new or updated permissions being added

None.

Description of how you validated changes

Unit tests cover all new enum values, feature flag on/off for both ECS and EKS,
userdataType behavior with launch templates (including the critical scenario
where the feature flag defaults to AL2023 with a launch template present), and
A1 instance validation. Build passes with 0 errors. 103 batch tests pass,
118 cx-api tests pass.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

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[aws-joelherd]

The pull request linter fails with the following errors:

❌ Features must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

Exemption Request

This change adds new enum values and a feature flag that only affects the default imageType value in the synthesized CloudFormation template. The feature flag is fully covered by unit tests (flag on/off for both ECS and EKS, userdataType with launch templates, A1 validation). The existing integration test for managed compute environments already covers the ECS_AL2023 image type. No new CloudFormation resource types are introduced.