[chore] Move JSON unmarshaling funcs to match other internal helpers

[bogdandrutu]

Extracted from #13450

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.40%. Comparing base (872a25e) to head (51ad6d9).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13470      +/-   ##
==========================================
- Coverage   91.44%   91.40%   -0.04%     
==========================================
  Files         533      532       -1     
  Lines       29564    29527      -37     
==========================================
- Hits        27034    26989      -45     
- Misses       1998     2004       +6     
- Partials      532      534       +2     

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