[bugfix] aws_rds_cluster: Fix inability to update serverlessv2_scaling_configuration.min_capacity argument from 1 to 0

[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

• Bug in aws_rds_cluster resource #44468 reported an issue where min_capacity could not be changed from 1 to 0.
• The function verify.SuppressMissingOptionalConfigurationBlock, configured as the DiffSuppressFunc for the serverlessv2_scaling_configuration block, is invoked not only for the block itself but also for its elements, including min_capacity.
  • This function suppresses the diff when the new value is 0 and the old value is 1, which exactly matches the behavior observed for min_capacity in this issue.
• To fix the issue, the function is now invoked only when the argument name is serverlessv2_scaling_configuration.#, which excludes min_capacity.
• Update the acceptance test to cover a case where min_capacity is changed from 1 to 0.
  • Without this modification, it is confirmed that the updated acceptance fails.

Relations

Closes #44468

Output from Acceptance Testing

$ make testacc TESTS='TestAccRDSCluster_serverlessV2Scaling' 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-fix_serverlessv2_scaling_min_capacity_from_1_to_0 🌿...
TF_ACC=1 go1.24.6 test ./internal/service/rds/... -v -count 1 -parallel 20 -run='TestAccRDSCluster_serverlessV2Scaling'  -timeout 360m -vet=off
2025/10/05 01:54:09 Creating Terraform AWS Provider (SDKv2-style)...
2025/10/05 01:54:09 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccRDSCluster_serverlessV2ScalingConfiguration
=== PAUSE TestAccRDSCluster_serverlessV2ScalingConfiguration
=== RUN   TestAccRDSCluster_serverlessV2ScalingRemoved
=== PAUSE TestAccRDSCluster_serverlessV2ScalingRemoved
=== CONT  TestAccRDSCluster_serverlessV2ScalingConfiguration
=== CONT  TestAccRDSCluster_serverlessV2ScalingRemoved
--- PASS: TestAccRDSCluster_serverlessV2ScalingRemoved (175.22s)
--- PASS: TestAccRDSCluster_serverlessV2ScalingConfiguration (295.63s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/rds        299.866s

[github-actions]

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