Fix document diagnostics with the pb33f JSON Schema fork

[daveshanley]

Use the pb33f JSON Schema fork based on upstream v6.0.3, with fixes for overwritten error locations and missing flattened reference errors. Use the published v6.0.3 tag without a local replacement, and update libopenapi to v0.41.3.

Document validation now reports invalid property names at the exact key, with the correct name, source location, and escaped path. It retains unrelated validation failures and omits generic error groups. Regression tests cover OpenAPI 3.1 and 3.2, repeated and escaped names, non-pattern constraints, boolean schemas, arrays, nested content constraints, and sibling failures.

Consumers that use JSON Schema types directly must change their imports to github.com/pb33f/jsonschema/v6; the fork has distinct Go type identities. The README describes this migration.

Validation: full tests and race tests pass; lint reports zero issues. Total coverage is 98.7%, and all three new diagnostic helpers have 100% statement coverage. Independent correctness and code-cleanup reviews found no remaining blockers.

Uses the fixes merged in pb33f/jsonschema#1. Replaces #315.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (4290880) to head (806500f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
- Coverage   98.36%   98.27%   -0.10%     
==========================================
  Files          79       80       +1     
  Lines        9433     9481      +48     
==========================================
+ Hits         9279     9317      +38     
- Misses        129      139      +10     
  Partials       25       25              

Flag	Coverage Δ
unittests	98.27% <100.00%> (-0.10%)	⬇️

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