Remove unused/non-useful fields from LLMFreeTextQuestionValidationResponse

[sjd210]

markCalculationInstructions has been unused for around a year and no longer exists in any content. It can be safely removed from the response DTO (and I also clean up the last remaining reference from the IsaacLLMFreeTextQuestion object while at it).

additionalMarkingInstructions is still used, but exclusively for adding per-question detail to the LLM prompt. We don't use it in the front-end and have no reason to send it in this response.

maxMarks is still used, but is now sent in the question DTO to be displayed in the front-end early. We therefore don't need to send a duplicate in this response later.

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To note: markBreakdown currently includes all mark descriptions so that these can be displayed in the front-end markscheme. We don't want them sent earlier in order to prevent users from seeing the markscheme without first answering the question, but this leads to a lot of redundancy in the database. I see two possible solutions to this, but both more complicated than simply removing fields as above:

• Send the mark descriptions in a separate response from the validation (this has difficulties with validating that both responses have arrived before displaying the markscheme so doesn't seem sensible)
• Sanitise the validation response to remove descriptions after sending it but before storing it in the database (although this means the database does not have an entirely accurate log of what was sent in that validation response to the user)
• OR just keep it as-is and don't worry about the redundancy

I think that the most sensible solution is probably to keep it for now, but have it in mind in the future if we start having database storage issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.30%. Comparing base (e2a11e6) to head (433fcc1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #728      +/-   ##
==========================================
+ Coverage   37.29%   37.30%   +0.01%     
==========================================
  Files         536      536              
  Lines       23726    23703      -23     
  Branches     2861     2861              
==========================================
- Hits         8849     8843       -6     
+ Misses      13995    13978      -17     
  Partials      882      882              

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