fix: Tries to fix the survey behavior

[AyakorK]

🎩 Description

Fixes duplicate answers on questionnaires that allow unregistered (non-logged-in) respondents.

The root cause was in session_token, used to identify anonymous respondents: it read session[:session_id], a key that is never actually assigned anywhere in the codebase. This made session_token return the same blank value for every anonymous visitor, so different respondents became indistinguishable from one another once answers were stored/exported, which is what looked like duplicates. It now uses request.session.id, the actual Rails session identifier, giving each anonymous visitor a stable, unique token.

On top of that:

• AnswerQuestionnaire now uses find_or_initialize_by instead of Answer.new, so a resubmission for the same respondent/question updates the existing answer instead of creating a second one.
• A uniqueness validation was added on Answer as a model-level safety net against duplicates, independent of the two fixes above.
• UserAnswersSerializer now captures question positions once at the start of an export instead of re-reading them live per answer, to avoid inconsistent column mapping if the questionnaire structure changes mid-export.

None of this changes the respondent-facing experience, same forms, same flow, just correct identification and data underneath.

There have also been separate, unconfirmed reports of answers appearing shifted/misaligned on submit and/or export. We could not pin down a cause for that with certainty. It's possible (not confirmed) that the session_token fix also alleviates part of it, since before this fix, every unregistered respondent shared the same blank identifier, meaning several different people's answers could end up merged under a single respondent row in an export, which could look like misalignment. This is a plausible side-effect at best, not a verified fix for that specific report.

📌 Related Issues

• Tries to fix https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=211285851&issue=OpenSourcePolitics%7Cintern-tasks%7C464

Tasks

• surveys_controller_extends.rb: fixed session_token/visitor_already_answered? to use request.session.id instead of the never-assigned session[:session_id]
• answer_questionnaire_extends.rb: switched to find_or_initialize_by instead of Answer.new to make answer submission idempotent
• answer_extends.rb: added a uniqueness validation on Answer as a model-level duplicate guard
• user_answers_serializer_extends.rb: cache question positions once at the start of export instead of re-reading them live per answer
• Add specs