`generate` does not exit with circular references

### tfplugingen-openapi CLI version

v0.3.0

### OpenAPI Spec File

```YAML
{
"openapi": "3.0.3",
"info": {
"title": "API",
"version": "1.0.0"
},
"paths": {
"/a": {
"post": {
"requestBody": {
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/A"
}
}
},
"required": true
},
"responses": {
"201": {
"description": ""
}
}
}
},
"/a/{id}": {
"get": {
"responses": {
"200": {
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/A"
}
}
},
"description": ""
}
}
},
"delete": {
"responses": {
"204": {
"description": "No response body"
}
}
}
}
},
"components": {
"schemas": {
"A": {
"type": "object",
"properties": {
"a": {
"$ref": "#/components/schemas/A"
},
"b": {
"$ref": "#/components/schemas/B"
}
}
},
"B": {
"type": "object",
"properties": {
"a": {
"$ref": "#/components/schemas/A"
},
"b": {
"$ref": "#/components/schemas/B"
}
}
}
}
}
}
```

### Generator Config

```YAML
provider:
name: foo
resources:
a:
create:
path: /a
method: POST
read:
path: /a/{id}
method: GET
delete:
path: /a/{id}
method: DELETE
```

### Expected Behavior

Create the results or display an error message.

### Actual Behavior

The tfplugin-openapi process did not exit, but ran continuously and printed error messages.

### Additional Information

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct