Strapi 5 | isCreatingEntry always false in useContentManagerContext
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## Bug report

### Required System information

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- Node.js version: 20
- NPM version: 8
- Strapi version: 5.6.0
- Database: PostgreSQL
- Operating system: Win 11 Pro
- Is your project Javascript or Typescript: Javascript

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### Describe the bug

When doing the following:
```
import { unstable_useContentManagerContext as useContentManagerContext, useNotification } from '@strapi/strapi/admin';
const { isCreatingEntry } = useContentManagerContext();
```

The  `isCreatingEntry`variable is always false. When I look into the code here: https://github.com/strapi/strapi/blob/146a31b564bc8232686331f6b28f7ff966817963/packages/core/content-manager/admin/src/hooks/useDocument.ts#L238
it checks if the id is `create`, which it is not. The id is undefined when you create an entry.

I injected a button with this code:
```
app.getPlugin('content-manager').injectComponent('editView', 'right-links', {
  name: `${pluginId}-get-link-button`,
  Component: GetLinkButton,
});
```

### Steps to reproduce the behavior

1. Create a simple plugin where you can log out the `isCreatingEntry` variable.
2. Go to any content type and try to create a new entry.
3. `isCreatingEntry` is always false

### Expected behavior

The `isCreatingEntry` should be true when I am in the process of creating a new entry.

### Screenshots

![image](https://github.com/user-attachments/assets/9afad33c-bd8b-4b04-80e1-7f4c696bfeba)


### Code snippets

If applicable, add code samples to help explain your problem.

### Additional context

Add any other context about the problem here.
