Consistently serialize child members returned from route handlers

[halter73]

The comments show the response bodies produced by the following route handlers before and after the PR.

var app = WebApplication.Create(args);

// Before: { "p": "p" }
// After: { "c": "c", "p": "p" }
app.MapGet("/task", () => Task.FromResult<Parent>(new Child()));
app.MapGet("/valuetask", ValueTask<Parent> () => new(new Child()));

// Before: { "c": "c", "p": "p" }
// After: { "c": "c", "p": "p" }
app.MapGet("/", Parent () => new Child());

// Before: { "c": "c", "p": "p" }
// After: { "c": "c", "p": "p" }
app.MapGet("/taskobj", () => Task.FromResult<object>(new Child()));
app.MapGet("/valuetaskobj", ValueTask<object> () => new(new Child()));

app.Run();

record Parent(string P = "p");
record Child(string C = "c") : Parent;

This shows an inconsistency in how returned objects are serialized in async methods that return a Task<Parent> or ValueTask<Parent> vs non-async methods that return Parent when the runtime type is a child type.

The non-async route handler has the right behavior because it results in a call to WriteAsJsonAsync<object>(...) instead of WriteAsJsonAsync<Parent>(...) which is what gets called in the async case. See https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-polymorphism

Fixes #39856

[davidfowl]

This change doesn't feel right to me but I understand the current inconsistency with the IResult type.