Execute one step of the draft and return what it produced

Runs a SINGLE step of the workflow's current DRAFT on a worker and returns its output. Nothing is persisted: no run row, no version, no step history — this is a rehearsal of one step, not a run.

The step really executes. It calls the third party for real, so testing a step that sends an email sends the email.

What a caller must know:

• Failure is reported inside a 200. A step that threw, a step whose piece rejected the config, and "no worker is available to run this right now" all come back as HTTP 200 with success: false and a human-readable error. Branch on success, never on the status code. A non-2xx here means the REQUEST was refused (bad id, no permission), not that the step failed.
• nodeId is the step's stable NAME as it appears in the definition tree (FlowAction.name), not a display name and not an index.
• prevOutputs seeds {{steps.<name>.…}} for this test; triggerData seeds {{trigger.…}}. Both are optional — without them the step resolves those expressions against nothing.
• The call blocks for up to 45 seconds waiting for a worker; it does not carry Idempotency-Key (a duplicate costs one wasted worker call and persists nothing).