Retry test result

Re-run a single test result in place, whatever its status. Clones the result into a new PENDING result on the same run (re-running the identical generated case), cancels the original so it drops out of the run's score average, re-opens a finished run (a canceled one included) so stats re-finalize, and dispatches execution per agent type. Supports all agent types. A stuck original is canceled the same way and stays canceled, whether it is still running (its runner died) or still pending (its dispatch was lost). Rejected (400) when a retry of the result is already in flight. Returns 201 with the new result's id and status, and its URL in the Location header.