Say what a new operator owes beyond its own code

### Task Summary

The Java and Python implementation guides stop at the operator's own logic. What a new operator has to satisfy afterwards is not written down anywhere: which annotation states a constraint on which kind of config property, that a standalone Python fragment is required and what it may assume, how the verification run is invoked and read, and where a run that cannot happen is recorded so it is not skipped silently.

`getOutputSchemas` appears in neither existing guide, although every operator must implement it, and a schema that disagrees with what the executor emits fails at run time rather than at compile time.

Step 27 of 27 in #8325. It describes the verification harness, so it comes after every step that builds one.