api: return 400 in case of non-existent config fields

[aler9]

No description provided.

[codecov]

Codecov Report

Merging #2425 (36f54bb) into main (0389d7a) will increase coverage by 0.10%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2425      +/-   ##
==========================================
+ Coverage   61.58%   61.68%   +0.10%     
==========================================
  Files         129      129              
  Lines       14476    14480       +4     
==========================================
+ Hits         8915     8932      +17     
+ Misses       4887     4877      -10     
+ Partials      674      671       -3     

Files	Coverage Δ
internal/conf/conf.go	75.86% <ø> (ø)
internal/conf/path.go	28.36% <ø> (ø)
internal/core/api.go	64.63% <100.00%> (+2.30%)	⬆️

... and 3 files with indirect coverage changes

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