feat(batch): switch default imageType to ECS_AL2023 for EC2 compute environments (under feature flag)

[alpin87]

Closes #37205

Reason for this change

Amazon Linux 2 (AL2) is reaching EOL in June 2026. AWS Batch compute environments that do not explicitly specify imageType currently default to ECS_AL2, which is deprecated. New environments should use ECS_AL2023 by default.

Description of changes

Introduced a new feature flag @aws-cdk/aws-batch:defaultEcsAL2023 that switches the default imageType fallback from ECS_AL2 to ECS_AL2023 in ManagedEc2EcsComputeEnvironment.

• Added feature flag BATCH_DEFAULT_ECS_AL2023 in cx-api/lib/features.ts
• Updated the fallback logic in managed-compute-environment.ts to use ECS_AL2023 when the flag is enabled
• Updated JSDoc @default comments on images and imageType properties
• Auto-generated FEATURE_FLAGS.md and recommended-feature-flags.json updated via build

A feature flag is used instead of a direct default change to avoid breaking existing stacks (behavior change in CloudFormation template).

Describe any new or updated permissions being added

None.

Description of how you validated changes

• Added two unit tests in managed-compute-environment.test.ts:
  • Feature flag OFF: verifies default imageType remains ECS_AL2 (backward compat)
  • Feature flag ON: verifies default imageType becomes ECS_AL2023
• All 232 existing aws-batch tests pass
  Note: global coverage threshold failure is a pre-existing issue in the repository, unrelated to this change.

[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.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[alpin87]

Exemption Request

This change introduces a feature flag (@aws-cdk/aws-batch:defaultEcsAL2023) that only affects the default value of imageType in the CloudFormation template when no explicit value is provided.

The behavior change is:

• Flag OFF (default): imageType falls back to ECS_AL2 — identical to the current behavior, no template diff
• Flag ON: imageType falls back to ECS_AL2023 — existing integration tests already cover ECS_AL2023 explicitly (e.g., line 935 in managed-compute-environment.test.ts)

An integration test would require a real AWS deployment and the change is fully covered by the two new unit tests added in this PR. Requesting an exemption from the integration test requirement.