fix(rds): proxy target group does not depend on database instances when using writer property for database cluster

[go-to-k]

Issue # (if applicable)

Closes #31304 .

Reason for this change

Proxy Target Group should depend on and wait for Aurora instances to be ready before creating CloudFormation resource. (see the issue).

Now, the dependency is added when using a legacy instanceProps, but not added when using a writer property.

https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-rds/lib/proxy.ts#L535-L539

(The cluster has CfnDBInstance directly when using the instanceProps, but it has AuroraClusterInstance with CfnDBInstance as defaultChild when using the writer. So the cluster doesn't have the CfnDBInstance directly in the latter case.)

Description of changes

Added the dependency when using a writer property instead of instanceProps.

Description of how you validated changes

Both unit and integ 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

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 3398fd6
• Result: SUCCEEDED
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