for_user error message

Cryptic error when for_user variable is underfined for question
```
Traceback (most recent call last):
File "/opt/runpy/runpy", line 89, in do_POST
run_code(response, run_req)
File "/opt/runpy/code_runpy_backend.py", line 245, in run_code
user_ctx[name] = maint_ctx[name]
KeyError: 's'
```

Start by locating code_runpy_backend.py and the run_code entry point referenced in the traceback, then reproduce a question with an undefined for_user variable. Trace how the KeyError is surfaced and define completion as a clearer error message for that case, supported by a regression test if the surrounding tests identify a suitable location.

Written by the indexing model from the issue text.