client-preset with nested fragment for union type may generates fragment types that use undefined fragment types

### Which packages are impacted by your issue?

@graphql-codegen/client-preset

### Describe the bug

When using client preset, codegen generates unexpected invalid type definitions for nested fragment reference (*) for union type. In particular, fragment type definitions for unused (non-referenced) types of union are not emitted, although they are used from another fragment type definitions.

(*) The fragments are such as:

```ts
// type User = Human | Robot

const HumanNameFragment = graphql(`
fragment HumanName_User on User {
... on Human {
name
}
}
`);

const TaskUserFragment = graphql(`
fragment TaskUser_User on User {
... on Human {
id
}
... on Robot {
id
}
...HumanName_User
}
`);

const TaskDetailFragment = graphql(`
fragment TaskDetail_Task on Task {
id
assignee {
...TaskUser_User
}
}
`);

const GetTaskDocument = graphql(`
query GetTask($id: String!) {
task(id: $id) {
...TaskDetail_Task
}
}
`);
```

### Your Example Website or App

https://github.com/jet2jet/graphql-code-generator-issue-fragment-type-undefined

### Steps to Reproduce the Bug or Issue

1. Clone the repository above
2. Run `npm ci` and `npm run codegen`
3. Run `npm run typecheck` to check type definitions

### Expected behavior

No error is reported for `npm run typecheck`

### Screenshots or Videos

_No response_

### Platform

- OS: Linux (WSL)
- NodeJS: 24.16.0
- `graphql` version: 16.14.2
- `@graphql-codegen/cli` version(s): 7.2.0
- `@graphql-codegen/client-preset` version(s): 6.1.3

### Codegen Config File

```ts
import { CodegenConfig } from "@graphql-codegen/cli";

const config: CodegenConfig = {
schema: "schema.graphql",
generates: {
'src/gql/': {
documents: ['src/**/*.mts', '!src/gql/**/*'],
preset: 'client',
config: {
nonOptionalTypename: process.env.WITH_TYPENAME === '1', // for test
},
},
},
};

export default config;
```

### Additional context

If `nonOptionalTypename` is set to true, all fragment types are emitted correctly, which results no error.

Start with the reproduction repository's schema.graphql, src/**/*.mts documents, and src/gql/ generated output. Run npm ci, npm run codegen, and npm run typecheck, then inspect the generated fragment type definitions for the nested union fragments. Done means the existing reproduction generates definitions for all referenced fragment types and npm run typecheck reports no errors.

Written by the indexing model from the issue text.