fix(rds): propagate RemovalPolicy.RETAIN_ON_UPDATE_OR_DELETE to helper resources

[suhas38222]

Issue

Closes #37780.

Reason for this change

RemovalPolicy.RETAIN_ON_UPDATE_OR_DELETE was not propagated to the helper resources (subnet groups, cluster instances) created by DatabaseCluster, DatabaseInstance, and ServerlessCluster. Three places in the RDS private utilities compared against RemovalPolicy.RETAIN only — RETAIN_ON_UPDATE_OR_DELETE silently fell through to DESTROY, leaving those child resources with no explicit deletion policy at all (CloudFormation default: Delete).

Description of changes

packages/aws-cdk-lib/aws-rds/lib/private/util.ts

• helperRemovalPolicy: extended the condition to pass RETAIN_ON_UPDATE_OR_DELETE through as-is instead of collapsing it to DESTROY. RetainExceptOnCreate is correct here: it still permits cleanup of the helper if its own creation fails during a rollback, while retaining it on normal updates and deletes.
• defaultDeletionProtection: extended the condition to also auto-enable deletion protection when removalPolicy is RETAIN_ON_UPDATE_OR_DELETE, consistent with the existing RETAIN behaviour.

packages/aws-cdk-lib/aws-rds/lib/serverless-cluster.ts

• Replaced the inline props.removalPolicy === RemovalPolicy.RETAIN ? props.removalPolicy : undefined check with renderUnless(helperRemovalPolicy(props.removalPolicy), RemovalPolicy.DESTROY), bringing it in line with DatabaseCluster and DatabaseInstance.

Describe any new or updated permissions being added

None. This is a pure bug fix with no IAM or AWS service interaction.

Description of how you validated changes

Unit tests (packages/aws-cdk-lib/aws-rds/test/):

• cluster.test.ts: added RETAIN_ON_UPDATE_OR_DELETE row to both existing test.each blocks that cover DeletionPolicy/UpdateReplacePolicy on the cluster, instance, and subnet group. Added a new test.each block verifying DeletionProtection is auto-enabled for both RETAIN and RETAIN_ON_UPDATE_OR_DELETE.
• instance.test.ts: same additions — RETAIN_ON_UPDATE_OR_DELETE row in the existing policy propagation test, plus a new DeletionProtection auto-enable test.
• serverless-cluster.test.ts: new test asserting the subnet group gets RetainExceptOnCreate when the cluster is created with RETAIN_ON_UPDATE_OR_DELETE.

Integration test (packages/@aws-cdk-testing/framework-integ/test/aws-rds/test/integ.cluster-instance-removal-policy.ts):

Creates a DatabaseCluster and a DatabaseInstance each with removalPolicy: RemovalPolicy.RETAIN_ON_UPDATE_OR_DELETE and verifies that the subnet groups and cluster instances inherit RetainExceptOnCreate. Snapshot generated locally via cdk synth.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES