fix: do not panic when encountering external schemas

[danielgtaylor]

This PR ensures the schema link transformer only runs when the ref is a valid intra-document (local) ref and not an external one. External ones are ignored.

Fixes #703.

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.78%. Comparing base (169da69) to head (227f5fc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #729   +/-   ##
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  Coverage   92.78%   92.78%           
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  Files          22       22           
  Lines        5062     5062           
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  Hits         4697     4697           
  Misses        314      314           
  Partials       51       51           

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