### 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 - [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md) ---- *By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*