BadRequestError: Error code: 400 - {'error': {'message': "Unsupported parameter: 'stop' is not supported with this model.", 'type': 'invalid_request_error', 'param': 'stop', 'code': 'unsupported_parameter'}}

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### Describe the bug

Using `openai.AzureOpenAI` with BERTopic for fine-tuning topic representations crashes due to an unsupported `stop` parameter being inserted.

### Reproduction

```python
from bertopic import BERTopic
from bertopic.representation import OpenAI
from openai import AzureOpenAI

client = AzureOpenAI(
azure_endpoint="https://super-secret.openai.azure.com/",
azure_deployment="deployment",
api_key="api_key",
api_version="2024-12-01-preview",
)

representation_model = OpenAI(client=client)
labels, _ = BERTopic().fit(docs)
```

This results in an HTTP 400 error from Azure:

```
BadRequestError: Error code: 400 - {
'error': {
'message': "Unsupported parameter: 'stop' is not supported with this model.",
'type': 'invalid_request_error',
'param': 'stop',
'code': 'unsupported_parameter'
}
}
```

### Workaround

I resolved the issue by monkey-patching [bertopic/representation/_openai.py#L220](https://github.com/MaartenGr/BERTopic/blob/master/bertopic/representation/_openai.py#L220) to remove the parameter:

```python
kwargs.pop("stop", None)
```

### BERTopic Version

0.17.3

Start in bertopic/representation/_openai.py around line 220, where the issue reports that the stop parameter is inserted for the AzureOpenAI request. Reproduce the BERTopic example with AzureOpenAI and verify the request no longer fails with the unsupported-parameter error. Done means the reported configuration works without requiring the monkey-patch workaround.

Written by the indexing model from the issue text.