CSP directives seems to be empty

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

5.4.0

### Plugin version

5.2.3

### Node.js version

22.19.0

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

15.6.1 (24G90)

### Description

I'm using the following plugins in my fastify app:
- `@fastify/swagger`
- `@fastify/swagger-ui`
- `@fastify/helmet`

In the documentation of `@fastify/swagger` it is stated that it can be integrated nicely with `@fastify/helmet`.

Based on my investigation the csp.json should be populated like this:
- There is a dedicated script called `prepare-swagger-ui.js` that should calculate the hashes of all the inline script and style tags that are used in the html files of the ui
- The script stores the hashes in the `/static/csp.json` file
- This file is totally empty, which means that there is something wrong with this script or it is not running when the package is published
- This also means that if one would like to access the `fastifyIntance.swaggerCSP` directives those are empty as well

### Link to code that reproduces the bug

Minimal code to reproduce the error:
``` npm install fastify @fastify/helmet @fastify/swagger @fastify/swagger-ui ```

```typescript
import fastifyHelmet from "@fastify/helmet";
import fastifySwagger from "@fastify/swagger";
import fastifySwaggerUi from "@fastify/swagger-ui";
import fastify from "fastify";

const app = fastify();

await app.register(fastifySwagger, {
openapi: {
info: {
title: "Test API",
description: "Test API",
version: "1.0.0",
},
},
});

await app.register(fastifySwaggerUi, {
routePrefix: "/docs",
});

console.log(app.swaggerCSP);
```

Result running this script:

```json
{ script: [], style: [] }
```

### Expected Behavior

When setting up the plugin the csp directives should be populated with the correct hashes.