Fix Swagger sort key fallback for empty endpoint tags metadata

[AleksaMCode]

Fixes Swagger generation failure when endpoint tags metadata is empty (for example WithTags() with no values or [Tags()]), which currently throws InvalidOperationException: Sequence contains no elements from the default sort key selector.

The issue or feature being addressed

Fixes #4124

Details on the issue fix or feature implementation

• Updated the default sort key selection in SwaggerGeneratorOptions to handle empty tags safely.
• Kept existing behavior for normal cases: when tags exist, the first tag is still used as the sort key.
• Added fallback chain for empty-tag scenarios:
  1. first tag (if present)
  2. ApiDescription.RelativePath
  3. ApiDescription.ActionDescriptor?.DisplayName
  4. empty string
• Added a test in SwaggerGeneratorTests: GetSwagger_DoesNotThrow_WhenTagsMetadataIsEmpty

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (eedf135) to head (09d555b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4125   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files         111      111           
  Lines        4115     4118    +3     
  Branches      843      844    +1     
=======================================
+ Hits         3918     3921    +3     
  Misses        197      197           

Flag	Coverage Δ
Linux	95.21% <100.00%> (+<0.01%)	⬆️
Windows	95.21% <100.00%> (+<0.01%)	⬆️
macOS	95.21% <100.00%> (+<0.01%)	⬆️

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[martincostello]

Thanks.

[github-actions]

Thanks for your contribution @AleksaMCode - the changes from this pull request have been published as part of version 10.3.0 📦, which is now available from NuGet.org 🚀