fix: instructor dashboard add info encoded in the request body

[marslanabdulrauf]

Description

This pull request improves how the only_if_higher flag is handled in the instructor API for rescoring learners. The main change is to consistently support reading the flag from both query parameters and the request body, matching how the frontend sends data. This also makes the code more robust by accepting both boolean and string values for the flag.

Improvements to request handling:

• Added a new _get_only_if_higher helper function to extract the only_if_higher flag from either query parameters or the request body, accepting both string and boolean representations.
• Updated the RescoreView endpoint and its documentation to clarify that only_if_higher can be provided in either the query string or request body.
• Replaced the previous direct query param check for only_if_higher in the post method with the new helper for consistent and reliable extraction.

Supporting Information

https://github.com/mitodl/hq/issues/11874 (MIT Internal)

Testing instructions

Steps to Reproduce

Make sure you have a course with graded sub-section with at least one problem(multi-select would do)

1. Enroll yourself as a learner in course.
2. Log in as the learner account.
3. Go to the unit page
4. Go to the first question "Multiple Choice" and choose "Correct Answer" - click submit.
5. You will see your score is reflected as correct
6. Log in as a staff member.
7. Go to the problem's unit in studio
8. Click the pencil icon to edit the that problem, "Multiple choice"
9. Scroll to Answers
10. Change the correct answer to be "Incorrect*"
11. Save your changes and publish the page.
12. Go to the grading module on the instructor dashboard
13. Type your learner account information into "Specify Learner", click "Select"
14. Put the location block for the problem into "Specify Problem", click "Select"
15. The page will update with the learner's responses to the problem.
16. Next to "Rescore submission" click "Rescore only if score improves", and confirm "rescore"
17. Reload the page and enter the learner name and problem block again.
18. The score should be same, 1/1
19. Log in as your learner and reload the unit with the problem
20. You should see that you are marked 1.0/1.0

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[AhtishamShahid]

This description can be updated to something like "Optional: If 'true', only update scores that are higher than current. " "May be provided as a query parameter or in the request body (JSON boolean or string)."

[AhtishamShahid]

It would be great if a unit test were added for this change