[MergeController] Add ProducesResponseType attributes, six tests

[imnasnainaec]

This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.69%. Comparing base (3c0dd96) to head (0dd33e3).
⚠️ Report is 28 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3844      +/-   ##
==========================================
+ Coverage   73.42%   73.69%   +0.26%     
==========================================
  Files         287      287              
  Lines       10773    10773              
  Branches     1344     1344              
==========================================
+ Hits         7910     7939      +29     
+ Misses       2468     2437      -31     
- Partials      395      397       +2     

Flag	Coverage Δ
backend	83.52% <ø> (+0.61%)	⬆️
frontend	66.03% <ø> (ø)

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[andracc]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)