fix: better validation of manual schemas

[danielgtaylor]

Manually created schemas were not having propertyNames and requiredMap set up, which meant that validation of the inputs for those manual schemas was incomplete. This fixes it by backfilling any pre-computed data during the PrecomputeMessages() call, ensuring the validation works as expected.

This is related to some more complex scenarios described in #185 where schemas can't be generated from structs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (57554dc) 93.41% compared to head (042c385) 93.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
+ Coverage   93.41%   93.44%   +0.02%     
==========================================
  Files          17       17              
  Lines        2172     2181       +9     
==========================================
+ Hits         2029     2038       +9     
  Misses        105      105              
  Partials       38       38              

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