[bugfix] aws_rds_cluster: Fix failure when modifying provisioned IOPS storage

[tabito-hara]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

• rds cluster storage iops change #44697 reported that when only the iops value of Provisioned IOPS storage (io1, io2) is updated, the following error occurs:

  When modifying Provisioned IOPS storage, specify a value for both allocated storage and iops.

  • As the error message indicates, when modifying parameters for Provisioned IOPS storage, both allocated_storage and iops must be provided to the API.
  • This PR fixes the issue by ensuring that both iops and allocated_storage are passed to the AWS API, even when only one of them is changed.
    • If storage_type is io1 or io2, both iops and allocated_storage are now passed to the AWS API.

• Without this modification, the newly added acceptance tests failed with the same error.

Relations

Closes #44697

Output from Acceptance Testing

$ make testacc TESTS='TestAccRDSCluster_(allocatedStorage|iops)' PKG=rds
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Running acceptance tests on branch: 🌿 b-aws_rds_cluster-modify_provisioned_iops_storage 🌿...
TF_ACC=1 go1.24.8 test ./internal/service/rds/... -v -count 1 -parallel 20 -run='TestAccRDSCluster_(allocatedStorage|iops)'  -timeout 360m -vet=off
2025/10/20 03:21:03 Creating Terraform AWS Provider (SDKv2-style)...
2025/10/20 03:21:03 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccRDSCluster_allocatedStorage_io1
=== PAUSE TestAccRDSCluster_allocatedStorage_io1
=== RUN   TestAccRDSCluster_allocatedStorage_gp3
=== PAUSE TestAccRDSCluster_allocatedStorage_gp3
=== RUN   TestAccRDSCluster_iops
=== PAUSE TestAccRDSCluster_iops
=== CONT  TestAccRDSCluster_allocatedStorage_io1
=== CONT  TestAccRDSCluster_iops
=== CONT  TestAccRDSCluster_allocatedStorage_gp3
--- PASS: TestAccRDSCluster_iops (1855.16s)
--- PASS: TestAccRDSCluster_allocatedStorage_gp3 (1884.70s)
--- PASS: TestAccRDSCluster_allocatedStorage_io1 (2427.52s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/rds        2431.880s

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