Nested fields of @computed parent are not resolved

### Issue workflow progress

_Progress of the issue based on the
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- [x] 1. The issue provides a reproduction available on Github, Stackblitz or CodeSandbox
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>
> Please make sure the GraphQL Tools package versions under `package.json` matches yours.
- [ ] 2. A failing test has been provided
- [ ] 3. A local solution has been provided
- [ ] 4. A pull request is pending review

---

**Describe the bug**

Schema (User Service):
```gql
type User @canonical {
id: ID!
name: String!
organizationId: String!
}
```

Schema (Organization Service):
```gql
type Organization {
id: ID!
name: String!
users: [User!]
}

type User @key(selectionSet: "{ id }") {
id: ID!
organization: Organization @computed(selectionSet: "{ organizationId }")
}
```

Query:
```gql
query DoesNotWork {
users(ids: ["1", "2"]) { # user service
name
organization { # organization service with @computed directive
name
users { # user service
name
}
}
}
}
```

We have two services (User, Organization). Inside the Organization service we want to extend the type `User` with the field `organization`. Therefore we have to use the `@computed` directive. The type returned by the directive (in our case `Organization`) includes fields that must be resolved by the User service. This does not work and throws following error: `Error: Cannot return null for non-nullable field User.name.`

**To Reproduce** Steps to reproduce the behavior:

Open [Sandbox](https://codesandbox.io/p/devbox/condescending-dewdney-zzk552) and execute the default query named "DoesNotWork" in GraphiQL.

**Expected behavior**

Nested fields combined with the `@computed` directive are always resolved correctly.

**Environment:**

- `@graphql-tools/schema": "^10.0.2"`
- `@graphql-tools/stitch": "^9.0.3"`
- `@graphql-tools/stitching-directives": "^3.0.0"`
- NodeJS: v20

Run the linked CodeSandbox and its default “DoesNotWork” query first to reproduce the failure. Then inspect @graphql-tools/stitch and @graphql-tools/stitching-directives, focusing on @computed resolution, and add a regression test for the demonstrated schema. Done means nested User fields under the computed Organization resolve without the non-null name error.

Written by the indexing model from the issue text.