User defined exception of `class: :forbidden` is swallowed by default implementation

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### AI Policy

- [x] I agree to follow this project's AI Policy, or I agree that AI was not used while creating this issue.

### Versions

Erlang/OTP 27 [erts-15.2.7] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit]
Elixir 1.18.4 (compiled with Erlang/OTP 27)
ash_json_api 1.4.38

### Operating system

MacOS

### Current Behavior

Attempting to create an implementation for an application specific error will not succeed if the `Splode.Error` class given is `:forbidden`.

The only real example here is to define an exception in accordance with the docs and observe that you're receiving a generic forbidden error instead of what you're trying to create.

### Reproduction

You should be able to verify it by attempting to create your own error and return if from an endpoint:
``` elixir
defmodule MyApp.Forbidden.NoSoupForYou do
@moduledoc false
use Ash.Error.Exception
use Splode.Error, fields: [], class: :forbidden

@impl Exception
def message(_error) do
"The request could not be processed because NO SOUP FOR YOU!"
end

defimpl AshJsonApi.ToJsonApiError do
@spec to_json_api_error(term) :: AshJsonApi.Error.t() | list(AshJsonApi.Error.t())
def to_json_api_error(error) do
%AshJsonApi.Error{
id: Ash.UUID.generate(),
status_code: 403,
code: "no_soup_for_you",
title: "No Soup For You Error",
detail: error.message
}
end
end
end
```

This is roughly the implementation that I found this on, and changing the class specified to e.g. `:invalid` gives the expected error.

### Expected Behavior

I'm not entirely sure. This seems intentional, but I would have either expected a warning, or that the implementation was respected. The issue seems to be an explicit pattern match on the `:forbidden` class:

https://github.com/ash-project/ash_json_api/blob/50474338c81f8cc9540555e9d07d980e8697e009/lib/ash_json_api/error/error.ex#L34-L44

I'm happy to work through a PR to address this in whatever fashion is desirable.