Add OpenAPI tests for the presence of meta

[bkoelman]

Adds test coverage for meta in OpenAPI schemas. Because it is a free-format dictionary, we can't generate statically typed properties for built-in keys such as total, requestBody, and stackTrace.

Closes #1063.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.10%. Comparing base (78e25fb) to head (263a848).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1996   +/-   ##
=======================================
  Coverage   92.10%   92.10%           
=======================================
  Files         438      438           
  Lines       15005    15005           
  Branches     2488     2488           
=======================================
  Hits        13821    13821           
  Misses        730      730           
  Partials      454      454           

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