fix: prevent overwriting schema validations

[danielgtaylor]

This PR prevents overwriting of schema validation fields when using a SchemaTransformer or other mechanism where parsing a struct field with tags could result in overwritten field values such as pattern. The various tag functions have been updated to take the current field value as the default rather than returning the zero value as default.

Fixes #557

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.76%. Comparing base (cce4569) to head (ca4b5f6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
- Coverage   92.78%   92.76%   -0.02%     
==========================================
  Files          22       22              
  Lines        3893     3886       -7     
==========================================
- Hits         3612     3605       -7     
  Misses        236      236              
  Partials       45       45              

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[danielgtaylor]

Slight coverage dip is just due to the number of removed lines, as I was able to clean up some of the code a bit.