test: support for_each acceptance state checks

[sionsmith]

Summary

• replace terraform-plugin-testing generic checks that cannot parse string-keyed for_each addresses
• inspect the resulting Terraform state directly so the concurrent role acceptance scenario remains fully asserted

Context

The live WorkOS apply succeeded for all concurrent roles after #29. The acceptance job failed only when the v1.6 test helper attempted to resolve addresses such as workos_organization_role.concurrent["admin"].

Testing

• go test -race ./internal/provider -run TestAccOrganizationRoleResource_Concurrent -count=1
• go vet ./internal/provider

Follow-up to #29.