fix: Support custom tag ordering and null assignment in OpenApiOperation.Tags

[Hardik K (hardikkaurani)]

Pull Request

Description

This PR brings OpenApiOperation.Tags behavior into parity with OpenApiDocument.Tags (which was updated in #3053 and #3054).

Currently, OpenApiOperation.Tags:

1. Silently ignores assignments of null, preventing callers from resetting the tags property to null.
2. Always wraps incoming sets in new HashSet<OpenApiTagReference>(value, OpenApiTagComparer.Instance) unless it was already a HashSet<OpenApiTagReference> using OpenApiTagComparer. This discards custom comparers, custom tag orderings (SortedSet<OpenApiTagReference>), and ImmutableSortedSet<OpenApiTagReference>.

This fix updates the setter to allow null and preserves custom comparers, SortedSet, and ImmutableSortedSet.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Related Issue(s)

Fixes #3115

Changes Made

• Updated OpenApiOperation.Tags setter to reset _tags = null when assigned null.
• Updated OpenApiOperation.Tags setter to preserve HashSet instances with non-default comparers, SortedSet<OpenApiTagReference>, and ImmutableSortedSet<OpenApiTagReference>, aligning with OpenApiDocument.Tags.
• Added conditional import using System.Collections.Immutable; under #if NET.
• Added unit tests in OpenApiOperationTests.cs verifying null re-initialization, custom equality comparers, sorted sets, and immutable sorted sets.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Local verification:

• dotnet run --project test/Microsoft.OpenApi.Tests/Microsoft.OpenApi.Tests.csproj -> 1304 passed, 0 failed.
• dotnet run --project test/Microsoft.OpenApi.Readers.Tests/Microsoft.OpenApi.Readers.Tests.csproj -> 635 passed, 0 failed.

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Versions applicability

• My change applies to the version 1.X of the library, if so PR link:
• My change applies to the version 2.X of the library, if so PR link:
• My change applies to the version 3.X of the library, if so PR link:
• I have evaluated the applicability of my change against the other versions above.

Additional Notes

cc Vincent Biret (@baywet) for review as this mirrors the previous improvements made to OpenApiDocument.Tags.

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