feat(ecs): add L2 support for native ECS blue/green deployments

[pahud]

Issue # (if applicable)

Reason for this change

This PR reverts the revert commit 86caea16c4 ("Revert "revert(ecs): add support for native blue/green deployments in ECS L2 (#35061) (#35170)") without changing the git history and mitigate the JSII compatibility issues with minimal changes.

The original PR #35061 added support for native blue/green deployments in ECS L2 but was reverted due to JSII compatibility issues reported in #35167. The issue was that adding an optional alternateTarget parameter to the attachToELBv2 method caused JSII breaking changes, as JSII treats any method signature change as breaking because it affects generated bindings in other languages (Python, Java, C#, Go).

Description of changes

This PR restores the blue/green deployment functionality from PR #35061 with minimal changes to address the reviewer feedback:

1. Simplified approach - Since attachToELBv2 is a private method, JSII doesn't process it for cross-language bindings, so adding an optional parameter is safe
2. Restored original implementation - Added the optional alternateTarget parameter back to the private attachToELBv2 method
3. Removed unnecessary complexity - Eliminated the separate attachToELBv2WithAlternateTarget method that was created due to overcautiousness about JSII compatibility
4. Fixed unused variable - Removed the unused resources variable in alternate-target-configuration.ts (leftover from previous PR)
5. Added regression tests - Ensured services without alternate targets continue to work correctly

Key changes:

• attachToELBv2(targetGroup, containerName, containerPort, alternateTarget?) - restored to original approach with optional parameter (safe for private methods)
• Simplified loadBalancerTarget method - directly calls attachToELBv2 with the optional parameter
• Cleaned up unused code from previous implementation

Describe any new or updated permissions being added

No new permissions are being added. This change restores the original functionality with a simplified implementation based on reviewer feedback.

Description of how you validated changes

1. Unit tests: All existing tests continue to pass (355/355 ECS service tests passed)
2. Regression tests: Added specific tests to ensure services without alternate targets work correctly for both ALB and NLB scenarios
3. Alternate target tests: All 6 alternate target configuration tests pass
4. JSII compatibility: Private method signature changes are safe since JSII doesn't process private methods for cross-language bindings

Test results:

• ✅ alternate-target-configuration.test.ts: 6/6 tests passed
• ✅ base-service.test.ts: 25/25 tests passed
• ✅ ECS service tests: 355/355 tests passed

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]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 751aa22
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[pahud]

Hi @aemada-aws

Thank you for the review. I just restored everything from #35061 and made minimal necessary changes to avoid potential regression. Let me know if there's any concerns.

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

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[aemada-aws]

@Mergifyio refresh

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[aemada-aws]

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