Fix tests breaking with libopenapi v0.38.0 lazy getters

[siretart]

libopenapi v0.38.0 introduced lazy GetSpecJSON getters which broke validation tests that relied on manually corrupting the struct fields.\n\nSince ValidateOpenAPIDocument falls back to evaluating the lazy getters, those tests fail because the manual corruption gets overwritten by the lazy JSON parser. Pre-warming the lazy parser before mutating the fields ensures the fallback path triggers as expected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.63%. Comparing base (14f2269) to head (1ccb290).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage   97.63%   97.63%           
=======================================
  Files          64       64           
  Lines        6991     6991           
=======================================
  Hits         6826     6826           
  Misses        132      132           
  Partials       33       33           

Flag	Coverage Δ
unittests	97.63% <100.00%> (ø)

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