JsonSerializerContext not working with minimalApi

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

After configure Web API service to use generated version of `JsonSerializerContext` for serializing JSON, still using default serialization

### Expected Behavior

Object returned by endpoint should be serialized using configured class derived from `JsonSerializerContext` with his configuration.

### Steps To Reproduce

```C#
using System.Text.Json;
using System.Text.Json.Serialization;

var builder = WebApplication.CreateSlimBuilder(args);

builder.Services.ConfigureHttpJsonOptions(options =>
options.SerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonSerializerContext.Default));

var app = builder.Build();

app.MapGet("/json", () => new Person("Alex"));
app.MapGet("/string", () => JsonSerializer.Serialize(new Person("Alex"),AppJsonSerializerContext.Default.Person));

app.Run();

record Person(string FirstName)
{
}

[JsonSerializable(typeof(Person))]
[JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.KebabCaseUpper)]
internal partial class AppJsonSerializerContext : JsonSerializerContext
{

}
```

endpoint `/string` (manually using AppJsonSerializerContext) returns:
```JSON
{"FIRST-NAME":"Alex"}
```

but endpoint `/json` returns default serialized JSON, without `AppJsonSerializerContext`:
```JSON
{
"firstName": "Alex"
}
```

### Exceptions (if any)

_No response_

### .NET Version

8.0.100

### Anything else?

Project configuration .csproj
```XML

net8.0
enable
enable
true

```

I've tried debugging generated code, endpoint `/string` goes into generated implementation, but `/json` not