Minimal Flow that writes and reads a persisted Attribute.
HTTP:
GET /primitives/attribute/start?workflowId=...&message=helloGET /primitives/attribute/read?workflowId=...reads the status, the payment progress, and the email from outside the Flow with oneget_attributescall. The read invokes no RPC and no Worker, and it also works on a closed run. The Flow never sets the email, so it returnsnull.