Skip IResult in metadata if it implements IEndpointMetadataProvider

[captainsafia]

Closes #57801, specifically #57801 (comment).

Current implementation assumes that any IResult type in metadata is one that implements IEndpointMetadataProvider to populate its value type, but that's not the case for all types that implement IResult.

Note, there is a case that doesn't work with this current check:

class MyResult : IResult, IEndpointMetadataProvider
{
  public string Content { get; set; }
  public PopulateMetadata()
  {
     builder.Metadata.Add(new ProducesResponseTypeMetadata(typeof(MyResult));
  }
}

But I'm inclined that scenarios like the above should be modeled like:

class MyResult<T> : IResult, IEndpointMetadataProvider
{
  public T Value { get; }
  public PopulateMetadata()
  {
     builder.Metadata.Add(new ProducesResponseTypeMetadata(typeof(T));
  }
}

[Copilot]

Pull Request Overview

This PR fixes an issue where custom IResult implementations were being incorrectly excluded from OpenAPI metadata generation. The fix distinguishes between built-in framework IResult types (that implement IEndpointMetadataProvider and handle their own metadata) and custom IResult implementations that should be included in API documentation.

• Updated the response metadata filtering logic to only skip IResult types that also implement IEndpointMetadataProvider
• Added test coverage for custom IResult implementations in API documentation
• Updated snapshot tests to reflect the new behavior with custom IResult types appearing in OpenAPI schemas

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Mvc/Mvc.ApiExplorer/src/ApiResponseTypeProvider.cs	Core fix - modified condition to only skip IResult types that implement IEndpointMetadataProvider
src/Mvc/Mvc.ApiExplorer/test/ApiResponseTypeProviderTest.cs	Added test for custom IResult return types with metadata
src/Mvc/Mvc.ApiExplorer/test/EndpointMetadataApiDescriptionProviderTest.cs	Added test for response metadata with custom IResult implementor
src/OpenApi/sample/Endpoints/MapSchemasEndpoints.cs	Added sample endpoint demonstrating custom IResult usage
Various snapshot files	Updated test snapshots to include new CustomIResultImplementor schema