OpenAPI XML comments: verify parameter binding-source handling for non-body parameter types

[khellang]

Background

In #64535 (fixing #67076) the XML comment operation transformer was changed so that <param> documentation only falls back to the request body when the parameter is actually bound to the body. The old code in XmlCommentGenerator.Emitter.cs had a narrow workaround that skipped only typeof(CancellationToken); it now uses a new IsRequestBodyParameter helper that checks ApiParameterDescription.Source == BindingSource.Body:

private static bool IsRequestBodyParameter(OpenApiOperationTransformerContext context, ParameterInfo? parameterInfo, XmlParameterComment comment)
{
    var modelNames = GetModelNames(parameterInfo, parameterInfo?.Name ?? comment.Name);

    foreach (var parameterDescription in context.Description.ParameterDescriptions)
    {
        if (parameterDescription.Source == BindingSource.Body
            && parameterDescription.Name is { } parameterName
            && modelNames.Contains(parameterName))
        {
            return true;
        }
    }

    return false;
}

That PR added negative tests for CancellationToken and [FromServices] (documentation for these parameters must not leak onto RequestBody.Description).

Follow-up

Now that the matching is keyed off ApiParameterDescription.Source/BindingSource instead of a hard-coded type check, we should systematically confirm and lock in the behavior for the rest of the special / non-body parameter types, since each takes a different binding path through ApiExplorer:

• HttpContext, HttpRequest, HttpResponse
• ClaimsPrincipal
• [FromKeyedServices]
• IFormFile / IFormFileCollection / [FromForm] — these do relate to the request body/form, so the description should land on the right place (form content), not be dropped
• [FromBody] parameters with custom names / IModelNameProvider

Concrete asks:

• Add negative tests for HttpContext, ClaimsPrincipal, and keyed services (controllers and Minimal APIs).
• Verify the positive case for form/file parameters still applies documentation correctly.
• Confirm ApiParameterDescription.Source is the right discriminator for every case above (e.g. BindingSource.Special, BindingSource.Services, BindingSource.FormFile, BindingSource.Form) and tighten IsRequestBodyParameter if any of them need explicit handling.

cc related: #64535, #67076