Student problem-set payload exposes reference_solution and full function_signature for every problem (incl. probeable oracle) #159

Summary

GET /api/problem-sets/<slug>/ returns reference_solution and function_signature for every problem, with no gating on the handler's display config. For probeable_code / probeable_spec the reference solution is the hidden oracle: a student can read it in browser devtools and skip probing entirely. function_signature (with parameter types) is likewise sent even when show_function_signature=False.

Code path

• purplex/problems_app/repositories/problem_set_membership_repository.py:148-153 — problem_data["function_signature"] = getattr(problem, "function_signature", None) and problem_data["reference_solution"] = getattr(problem, "reference_solution", None), unconditionally.
• purplex/problems_app/services/student_service.py:157-158 — copied verbatim.
• purplex/problems_app/views/student_views.py:78-82 — ProblemSetDetailView.get returns it as data["problems"].

Who is affected

Handlers that set show_reference_code: False in get_problem_config yet still receive the field: probeable_code (handlers/probeable_code/handler.py:207), probeable_spec (:275), debug_fix (:162), prompt (:213), mcq (:275). EiPL and Refute set it True and legitimately consume it (ProblemSet.vue:393,600), so a blanket removal would break them.

Suggested fix

Gate in the repository/service layer: only include reference_solution when the problem's handler display config has show_reference_code: True, and only include function_signature when show_function_signature is True (or strip types, matching what the handlers already do for the probe panel). Add an integration test per handler asserting presence/absence in the student payload. Audit ProblemSerializer/other student-facing serializers for the same fields.

Found while researching #135.