application/vnd.foo+json disables problem details

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

When you make a request using an `Accept` header that uses the `json` suffix, for example `application/vnd.foo+json`, this disables problem details, despite the requested output format being json.

### Expected Behavior

Problem details should not be disabled

### Steps To Reproduce

1. Create a minimal API project with the following code:
```cs
var builder = WebApplication.CreateSlimBuilder(args);
builder.Services.AddProblemDetails();

var app = builder.Build();
app.UseStatusCodePages();
app.MapPost("/hello", () => TypedResults.Ok());
app.Run();
```
2. Run `curl http://localhost:5182/hello -H "Accept: application/vnd.foo+json"`
3. Observe curl prints `Status Code: 405; Method Not Allowed` instead of `{"type":"https://tools.ietf.org/html/rfc9110#section-15.5.6","title":"Method Not Allowed","status":405}`

### Exceptions (if any)

N/A

### .NET Version

9.0.100-preview.7.24407.12

### Anything else?

ASP.NET Core version: 8.0 I think? My TFM is `net8.0`
IDE: VSCode 1.92.2
```
> dotnet --info
.NET SDK:
Version: 9.0.100-preview.7.24407.12
Commit: d672b8a045
Workload version: 9.0.100-manifests.baed1e37
MSBuild version: 17.12.0-preview-24374-02+48e81c6f1

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.100-preview.7.24407.12\

.NET workloads installed:
Configured to use loose manifests when installing new manifests.
There are no installed workloads to display.

Host:
Version: 9.0.0-preview.7.24405.7
Architecture: x64
Commit: static

.NET SDKs installed:
9.0.100-preview.7.24407.12 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.32 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.0-preview.7.24406.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.32 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.33 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.0-preview.7.24405.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.32 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.33 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.0-preview.7.24405.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```

For context, I am currently implementing https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/PackageRegistry/Registry.md, which uses a custom `Accept` header for API versioning, but requires problem details responses for any error.

Reproduce the issue with the minimal API using AddProblemDetails, UseStatusCodePages, MapPost, and the application/vnd.foo+json Accept header. Trace how problem-details content negotiation handles the json suffix, then add coverage for this request and verify that the 405 response uses the expected JSON problem-details body.

Written by the indexing model from the issue text.