tests: use mock servers for schema loading

[HNO3Miracle]

Make the OpenAPI 3.0 and 3.1 schema source URLs replaceable within package tests, then point the remote-schema cases at their existing httptest servers. This makes the assertions validate the supplied remote content instead of depending on GitHub network access. Defaults remain unchanged for library users.

Tested with go test ./....

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (f309f59) to head (03f8429).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   98.27%   98.26%   -0.01%     
==========================================
  Files          75       75              
  Lines        9192     9190       -2     
==========================================
- Hits         9033     9031       -2     
  Misses        132      132              
  Partials       27       27              

Flag	Coverage Δ
unittests	98.26% <100.00%> (-0.01%)	⬇️

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