Fix handling of duplicate enum values for dictionaries

[martincostello]

Pull Request

The issue or feature being addressed

Resolves #2956.

Details on the issue fix or feature implementation

Add Distinct() to remove repeated declarations of the same enum values to avoid ArgumentException.

Supersedes #3742.

[Copilot]

Pull request overview

Fixes a Swagger schema generation regression when dictionary keys are enums that contain multiple named members with the same underlying value (enum aliases), preventing ArgumentException during ToDictionary key materialization.

Changes:

• Deduplicate serialized enum key representations via Distinct() when building known dictionary keys in JsonSerializerDataContractResolver.
• Add a regression test covering dictionary schema generation for enums with duplicate underlying values.
• Add a new test fixture enum that includes duplicate underlying values (alias member).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/Swashbuckle.AspNetCore.TestSupport/Fixtures/Enums.cs	Adds an enum fixture with aliased/duplicate underlying values for regression coverage.
test/Swashbuckle.AspNetCore.SwaggerGen.Test/SchemaGenerator/JsonSerializerSchemaGeneratorTests.cs	Adds a regression test asserting duplicate enum values don’t break dictionary schema generation.
src/Swashbuckle.AspNetCore.SwaggerGen/SchemaGenerator/JsonSerializerDataContractResolver.cs	Applies Distinct() to enum key JSON values to avoid duplicate dictionary keys.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (c6a3c78) to head (889e256).
⚠️ Report is 80 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3839      +/-   ##
==========================================
+ Coverage   94.67%   94.95%   +0.28%     
==========================================
  Files         111      111              
  Lines        3868     3886      +18     
  Branches      780      783       +3     
==========================================
+ Hits         3662     3690      +28     
+ Misses        206      196      -10     

Flag	Coverage Δ
Linux	94.95% <100.00%> (+0.28%)	⬆️
Windows	94.95% <100.00%> (+0.28%)	⬆️
macOS	94.95% <100.00%> (+0.28%)	⬆️

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