Retries a step by creating a new execution and resuming the run from that point

The retry applies to the step's latest attempt: if the step has already been retried,
the request is forwarded to the most recent attempt in its retry chain.

• If the latest attempt is terminal (succeeded, failed, or skipped), a new
  execution is created that reuses that attempt's inputs and the run resumes.
• If the latest attempt is still in progress, that attempt is returned and no new
  execution is created, so the request is safe to repeat.

Only terminal steps can be retried: a 400 error is returned when the step has no
newer attempt and is not yet terminal.