fix(rds): DatabaseCluster.instanceEndpoints doesn't include writer endpoint

[badmintoncryer]

Issue # (if applicable)

Closes #29279.

Reason for this change

DatabaseCluster.instanceEndpoints should include writer's endpoint but doesn't.

Description of changes

Add writer's endpoint to DatabaseCluster.instanceEndpoints

Description of how you validated changes

Add unit tests

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 has failed. See the aws-cdk-automation comment below for failure reasons. If you believe this pull request should receive an exemption, please comment and provide a justification.

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[badmintoncryer]

Exemption Request
I think this modification doesn't need to add integ test.

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[paulhcsun]

Thanks for the fix @badmintoncryer!

[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).

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: e3468e7
• Result: SUCCEEDED
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[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).

[steffenjambit]

Thanks a lot for the fix @badmintoncryer!

[badmintoncryer]

@steffenjambit You're welcome!!!