fix: serialize organization role mutations

[sionsmith]

Summary

• serialize organization-role create, update, and delete requests per organization to prevent WorkOS priority-list races
• preserve concurrency between different organizations and honor context cancellation while waiting
• add HTTP-level concurrency regression coverage and a Terraform acceptance test matching the reported for_each scenario

Triage

This is a provider bug. Organization role mutations operate on an organization-scoped priority order, while Terraform creates independent role resources concurrently. The provider previously sent those mutations without coordination.

The fix uses organization-scoped serialization instead of retrying create requests. POST is not documented as idempotent, so an automatic retry could duplicate or incorrectly adopt a role after partial server-side success.

Testing

• go test -race ./...
• go vet ./...
• go build ./...
• go generate ./... (clean diff)
• go run github.com/golangci/golangci-lint/cmd/golangci-lint@v1.64.8 run ./...
• focused organization-role race tests passed 10 consecutive runs

A live WorkOS acceptance regression is included but was not run locally because WORKOS_API_KEY is not available in this environment.

Closes #28