$ref fragments are expected to be full openapi documents

### What are you generating using Kiota, clients or plugins?

API Client/SDK

### In what context or format are you using Kiota?

Nuget tool

### Client library/SDK language

Csharp

### Describe the bug

Kiota will throw an error if a referenced fragment isn't a full openapi document

```
fail: Kiota.Builder.KiotaBuilder[0]
OpenAPI error: (null) - [File: fragment.json] Version node not found.
```

### Expected behavior

fragments do not need to be full openapi documents and can be just a fragment of it

### How to reproduce

`dotnet kiota generate -l CSharp -d `

### Open API description file

```jsonc
{
"openapi": "3.0.1",
"info": {
"title": "Test API",
"version": "v1"
},
"paths": {
"/foo": {
"$ref": "fragment.json#/get"
}
}
}
```

**fragment.json**
```jsonc
{
"get": {
"responses": {
"200": {
"description": "Successful response",
"content": {
"application/json": {
"schema": {
"type": "object",
"properties": {
"message": {
"type": "string"
}
}
}
}
}
}
}
}
}
```

### Kiota Version

1.29.0

### Latest Kiota version known to work for scenario above?(Not required)

_No response_

### Known Workarounds

_No response_

### Configuration

_No response_

### Debug output

Click to expand log
```

```

### Other information

_No response_

Start with the `dotnet kiota generate -l CSharp -d ` entry point and reproduce the failure using the OpenAPI description and `fragment.json` shown in the issue. Trace external `$ref` handling to see why `fragment.json#/get` is treated as a complete document; done means the sample generates successfully without the Version node error.

Written by the indexing model from the issue text.