Include exception in ProblemDetailsContext for controller error responses

[BloodShop]

Summary

When using ExceptionHandlerMiddleware with controllers and CustomizeProblemDetails, the ProblemDetailsContext.Exception is always null. The same setup with Minimal APIs correctly populates the exception.

Reproduction

builder.Services.AddControllers();
builder.Services.AddProblemDetails(options =>
{
    options.CustomizeProblemDetails = context =>
    {
        // context.Exception is always null with controllers!
        if (context.Exception is not null)
        {
            context.ProblemDetails.Detail = context.Exception.Message;
        }
    };
});

app.UseExceptionHandler();
app.MapControllers();

[ApiController]
[Route("[controller]")]
public class FailController : ControllerBase
{
    [HttpGet]
    public ActionResult Get() => throw new Exception("BOOM!");
}

context.Exception is always null even though the exception is available.

Root Cause

Two different code paths handle problem details writing:

Minimal APIs (DefaultProblemDetailsWriter): Calls CustomizeProblemDetails directly with the original ProblemDetailsContext from the middleware — which already has the Exception set. Works correctly.

Controllers (DefaultApiProblemDetailsWriter → DefaultProblemDetailsFactory): The factory's ApplyProblemDetailsDefaults creates a new ProblemDetailsContext for the callback, but never sets the Exception property:

// Before: exception is lost
_configure?.Invoke(new() { HttpContext = httpContext!, ProblemDetails = problemDetails });

The Fix

In DefaultProblemDetailsFactory.ApplyProblemDetailsDefaults, resolve the exception from IExceptionHandlerFeature (which the ExceptionHandlerMiddleware sets on the HttpContext) and include it in the context:

// After: exception is propagated
var exception = httpContext?.Features.Get<IExceptionHandlerFeature>()?.Error;
_configure?.Invoke(new() { HttpContext = httpContext!, ProblemDetails = problemDetails, Exception = exception });

When no IExceptionHandlerFeature is present (validation errors, manual status codes, etc.), exception is null — same behavior as before.

Changes

DefaultProblemDetailsFactory.cs

• Read exception from IExceptionHandlerFeature on HttpContext.Features
• Pass it to the ProblemDetailsContext in the customize callback

Microsoft.AspNetCore.Mvc.Core.csproj

• Add reference to Microsoft.AspNetCore.Diagnostics.Abstractions (for IExceptionHandlerFeature)

ProblemDetailsFactoryTest.cs (tests)

• Verify exception is propagated when IExceptionHandlerFeature is present
• Verify exception is null when no feature is present

Behavior After Fix

Scenario	Before	After
Controller + ExceptionHandler	context.Exception == null ❌	context.Exception == <thrown> ✅
Minimal API + ExceptionHandler	context.Exception == <thrown> ✅	unchanged ✅
Controller, no exception	context.Exception == null	unchanged ✅
Validation errors	context.Exception == null	unchanged ✅

Breaking Changes

None. The only change is that ProblemDetailsContext.Exception is now populated where it was previously null. Code that checked for context.Exception is not null will now correctly enter that branch.

Fixes #65697

[dotnet-policy-service]

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