OpenAIGenerator uses chat_completions endpoint. Error with model that has no chat_template in config

Describe the bug
I'm using the OpenAIGenerator to access a vLLM endpoint on runpod. When using a base model like Mistral v0.3 that has not been instruction tuned and so does not have a chat template in it's config for the tokenizer, I get an error returned from the api endpoint. Digging into this I see that the OpenAIGenerator uses the chat_completion/ endpoint for the OpenAIGenerator and not the completion/ endpoint. This means I've been unintentionally using a chat template with other models up to this point.

Error message
"Cannot use apply_chat_template() because tokenizer.chat_template is not set and no template argument was passed!"

Expected behavior
I expected for the completions/ api endpoint to be used and the hugging face model to not try to use apply_chat_template()

Additional context
I tried to use the client.completions method directly as a work around.

completion = generator.client.completions.create(model=generator.model, prompt="And then, something unexpected happened.", **generator.generation_kwargs)

The process on the server crashes with a 'NoneType' object has no attribute 'headers'.

System:

• Haystack version (commit or version number): haystack-ai==2.2.0