Fix copy_to_regions optional field causing Value Conversion Error

[TJM]

Summary

• copy_to_regions was typed as []types.String in the schema struct but declared as a SetAttribute in the resource schema. When the field is omitted from config, the Terraform framework sets it to an unknown value that a plain Go slice cannot represent — only types.Set can.
• Changed CopyToRegions field from []types.String to types.Set and updated all conversion logic accordingly.
• Added an acceptance test covering creation without copy_to_regions set.

Reproduction

resource "couchbase-capella_cloud_snapshot_backup_schedule" "example" {
  interval        = 4
  retention       = 168
  start_time      = "2024-01-01T00:00:00+00:00"
  cluster_id      = couchbase-capella_cluster.example.id
  project_id      = couchbase-capella_cluster.example.project_id
  organization_id = local.capella_org_id
}

Previously produced:

│ Error: Value Conversion Error
│ Path: copy_to_regions
│ Target Type: []basetypes.StringValue
│ Suggested Type: basetypes.SetValue

Test plan

• TestAccSnapshotBackupScheduleResourceWithoutCopyToRegions — new test, creates resource without copy_to_regions
• TestAccSnapshotBackupScheduleResource — existing happy-path test still passes
• TestAccSnapshotBackupScheduleResourceInvalidCopyToRegions — existing negative test still passes

🤖 Generated with Claude Code

[TJM]

A workaround for anyone trying to set these up before this MR is merged, you can set copy_to_regions to an empty set:

  copy_to_regions = toset([])