chore(ec2-alpha): check IPAM Pool local in Operating Region only if specified

[Y-JayKim]

Issue # (if applicable)

Closes #34179

Reason for this change

The current implementation throws an error when an IPAM pool's locale is not in the operating regions, even though locale is an optional field in CloudFormation. However, according to AWS documentation, while locale is optional, if it is specified it must be one of the operating regions.

Description of changes

• Kept the locale validation check conditional on poolOptions.locale being provided, since locale is an optional field
• Changed the error message to be more descriptive about the requirement that locale must be configured as an operating region when specified
• Maintained strict validation when locale is provided to match AWS CloudFormation's behavior where it will error if the locale is not in operating regions
• Updated code comments to better reflect AWS's requirements

The key design decision was to keep the error throw (rather than changing to a warning) because:

1. CloudFormation will fail if locale isn't in operating regions
2. It's better to catch this at the CDK level with a clear error message
3. This matches AWS's documented behaviour

Describe any new or updated permissions being added

No new or updated IAM permissions are required for this change.

Description of how you validated changes

The changes maintain the existing behavior when locale is provided but improve the error message clarity. No new test cases were needed as this is a refinement of error handling rather than a functional change.

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

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 97cd0f1
• Result: SUCCEEDED
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