Check which NPDB-required practitioner fields are present

Validates that all practitioner fields required by the tenant's NPDB pipeline configuration are present and non-empty on the workflow identified by workflowId. Requires JWT permission CREDENTIALING_WORKFLOW_PIPELINE_TRIGGER. Returns the validation result without triggering the pipeline — use this endpoint before calling POST /pipeline-runs to confirm the workflow is ready. The set of required fields is tenant-configured. Returns 200 with a NpdbRequiredFieldsCheckResponse indicating which fields pass and which are missing. Returns 400 if the workflow is missing a linked practitioner.