Setting the Description in a ProducesResponseTypeAttribute works correctly for Minimal API

[sander1095]

Setting the Description in a ProducesResponseTypeAttribute works correctly for Minimal API

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Using ProducesResponseType with the Description property works with Minimal API

Description

This PR fixes the issue where the description is not set in a Minimal API application:

app.MapGet("/weatherforecast",
[ProducesResponseType<IEnumerable<WeatherForecast>>(StatusCodes.Status200OK, Description = "The service is healthy.")]
() =>
{
    return new Something();
}

This did work for Controllers AND for Minimal API's when the method body was empty. The tests sadly did not cover scenarios where the method body did have content.

This PR adds test cases in several places to check that the Description is added correctly to the OpenAPI document. It also fixes the underlying issues.

Please perform a careful review of the tests in the ApiExlorer project! I made some changes and am not 100% sure that the tests are 100% valid and represent real scenarios.

Fixes #60518

[dotnet-policy-service]

Thanks for your PR, @sander1095. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.