Fix document URL serialization

[martincostello]

Pull Request

The issue or feature being addressed

#3772 (comment)

Details on the issue fix or feature implementation

Fix URLs not being serialized correctly if a custom JsonSerializerOptions is provided.

[Copilot]

Pull request overview

This PR fixes a bug where document URLs were not being serialized correctly when custom JsonSerializerOptions were provided to SwaggerUI. The issue stemmed from incorrect use of the null-coalescing assignment operator (??=) in the previous implementation.

Changes:

• Fixed the RespondWithDocumentUrls method to correctly branch between custom and default JSON serialization options
• Simplified the code by replacing if-else logic with a ternary operator
• Ensured proper type conversion when using the source-generated JSON context

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (3d260bb) to head (ebc295e).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3773   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         111      111           
  Lines        3886     3886           
  Branches      783      783           
=======================================
  Hits         3689     3689           
  Misses        197      197           

Flag	Coverage Δ
Linux	94.93% <100.00%> (ø)
Windows	94.93% <100.00%> (ø)
macOS	94.93% <100.00%> (ø)

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