Invalid state with Github provider (err 500)

### 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

4.26.2

### Plugin version

7.8.0

### Node.js version

20.5.1

### Operating system

macOS

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

14.4.1

### Description

Hello,

While using the Github provider, I noticed something strange.

When setting the startRedirectPath, if you put a / in the path, it breaks the Github provider.

So, for example, if you set the startRedirectPath to /login/github and try to connect, you will get a result from the callback that looks like this
```json
{
"statusCode": 500,
"error": "Internal Server Error",
"message": "Invalid state"
}
```

But, setting the startRedirectPath to something like /github instantly works, so I think there's a bug somewhere.

Maybe worth to know, I use the autoload to load all my routes like that :
```ts
fastify.register(autoLoad, {
dir: path.join(__dirname, "src/routes"),
options: { prefix: "/api" },
});
```

From what I can see, it looks like the state and stateCookie variables are not equal in the defaultCheckStateFunction function in the index.js of the package. But I have no idea why.

### Steps to Reproduce

```ts
fastify.register(oAuthPlugin, {
name: "oauthName",

credentials: {
client: {
id: "xxx",
secret: "xxx",
},
auth: oAuthPlugin.GITHUB_CONFIGURATION,
},

startRedirectPath: "/login/github",
callbackUri: "http://localhost:3000/api/auth/providers/github/callback",
});
```

### Expected Behavior

_No response_