fix(ec2): set correct 'domain' property for CfnEIP in NAT Gateway

[SokiKawashima]

Issue # (if applicable)

Closes #33469

Reason for this change

The current implementation of CfnEIP for NAT Gateway in ec2-alpha uses props.vpc?.vpcId for the domain property. However, per the AWS CloudFormation Documentation, the domain must be set to the string "vpc" for VPC Elastic IPs.

Description of changes

• Code Changes:

let eip = new CfnEIP(this, 'EIP', {
  domain: 'vpc',
});

• Design Decisions:
  • Followed AWS documentation standards to ensure compatibility.
  • Maintained backward compatibility by limiting the fix to VPC-associated EIPs only.
• AWS Use Cases Enabled:
  • Supports proper NAT Gateway creation with VPC EIPs.
  • Aligns ec2-alpha module with AWS CloudFormation standards.

Describe any new or updated permissions being added

No new permissions are required.

Description of how you validated changes

No additional tests were introduced.

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

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: f3eccfe
• Result: FAILED
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