if a resource's `type` is `subscription`, the generated schema conflicts with the subscription schema

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### Versions

elixir 1.18.4
ash 3.6.2
ash_graphql 1.8.3

### Operating system

Linux docker container on macOS

### Current Behavior

I haven't verified all different potential iterations of this bug, but I have an application that does _not_ make use of graphql subscriptions. I recently added a resource modeling a "subscription" within my application, and named its graphql type `:subscription`. I noticed that in the generated graphql schema file, the schema now had a subscription key in the root schema, like this:

```graphql
schema {
mutation: RootMutationType
query: RootQueryType
subscription: Subscription
}
```

with `type Subscription` having my subscription resource's fields

where it had previously (and still does if I change the type name to anything other than `subscription`) looked like this:

```graphql
schema {
mutation: RootMutationType
query: RootQueryType
}
```

I assume this is because `Subscription` is effectively a reserved type name for graphql subscriptions. I'm not sure what would happen here if this occurred in an application that _did_ use graphql subscriptions, but I suspect these would overlap in a more catastrophic way (my application did still compile).

### Reproduction

- create a resource called `MyApp.Domain.Subscription` with any attributes
- it should use the `AshGraphql.Resource` extension, have a graphql `type` of `:subscription`, and wire up a query of some sort so it does show up in the schema file
- generate the sdl file
- see the behavior described

### Expected Behavior

I think the easiest way to avoid this would be to simply raise if a graphql type is called `subscription`, since that type name is effectively reserved by ash_graphql.

The name of the internal subscription type could also be made configurable, which would allow users to name a type `subscription` if they explicitly rename the internal subscription type.

The issue names no files or tests; reproduce it with a resource whose AshGraphql type is :subscription, then trace the generated SDL and the handling of the internal Subscription type. Done means a resource type named subscription no longer creates a conflicting root subscription schema, with behavior covered by a regression test.

Written by the indexing model from the issue text.