304: NewValidator - fix document model mutation

[litteratum]

Fix NewValidator's mutations for the document model.
It is just a guard around go-yaml.

The fix - clone YAML Node unconditionally in renderYAMLNodeForValidation.
I created a separate validator_model_mutation_test.go that tests all different combinations.

All tests pass.

The extra clone should not be a problem for performance because it is mostly done on initialization.

Fixes #304.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (f309f59) to head (449d178).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
- Coverage   98.27%   98.27%   -0.01%     
==========================================
  Files          75       75              
  Lines        9192     9191       -1     
==========================================
- Hits         9033     9032       -1     
  Misses        132      132              
  Partials       27       27              

Flag	Coverage Δ
unittests	98.27% <100.00%> (-0.01%)	⬇️

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