Fixes#13527.
When upgrading to [Next.js 15.5](https://nextjs.org/blog/next-15-5) with
Payload, you might experience a runtime or build error similar to this:
```ts
Type error: Type 'typeof import("/src/app/(payload)/api/graphql/route")' does not satisfy the expected type 'RouteHandlerConfig<"/api/graphql">'.
Types of property 'OPTIONS' are incompatible.
Type '(request: Request, args: { params: Promise<{ slug: string[]; }>; }) => Promise<Response>' is not assignable to type '(request: NextRequest, context: { params: Promise<{}>; }) => void | Promise<void> | Response | Promise<Response>'.
Types of parameters 'args' and 'context' are incompatible.
Type '{ params: Promise<{}>; }' is not assignable to type '{ params: Promise<{ slug: string[]; }>; }'.
Types of property 'params' are incompatible.
Type 'Promise<{}>' is not assignable to type 'Promise<{ slug: string[]; }>'.
Property 'slug' is missing in type '{}' but required in type '{ slug: string[]; }'.
```
This is because Next.js route types are now _stricter_. Our REST handler
is nested within a catch-all `/api/[...slug]` route, so the slug param
_will_ exist in the handler—but the _same_ handler is re-used for the
`/api/graphql` OPTIONS route, which **_is not_** nested within the
`slug` param and so it **_will not_** exist as the types suggest.
---
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---------
Co-authored-by: Jacob Fletcher <jacobsfletch@gmail.com>
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