Add a CaptureStartupErrors overload with RequestDelegate.

## Background and Motivation

Allow to return Json or a custom error page on startup errors.

## Proposed API
```

.CaptureStartupErrors((context, exception) =>
{
context.Response.StatusCode = StatusCodes.Status500InternalServerError;
return context.ExecuteObjectResultAsync(exception);
})
```

The issue names CaptureStartupErrors but does not identify a file, test, or entry point. Locate the existing API and its related tests, then compare the proposed RequestDelegate overload and verify that startup errors can produce JSON or a custom error page as shown.

Written by the indexing model from the issue text.