Generate and store a Board Certification PDF for a workflow

Generates a Board Certification PDF document from the board certification data on the credentialing workflow identified by id and stores it in cloud storage. Authenticate with the X-Credentialing-Api-Key header (API key), not JWT — typically invoked from Cloud Tasks. Returns 200 with the PDF storage URL when generation succeeds. Returns 204 with no body when there are no board certifications on the workflow to generate a PDF from.