Validation for custom_content is missing from gpt4 handler

[roman-romanov-o]

If you pass custom_content (image attachments) and image content parts into gpt-4 handler, no error will be raised, image will be just ignored.
That's bad UX.

[adubovik]

Be careful to check only the DIAL extensions to the OpenAI API - e.g. custom_content and image content parts with DIAL URLs.

The image content parts with regular URLs may be just fine, since we really don't know the capabilities of the model, which fall into the catch-all-gpt4 category. It may be GPT-4.5 that supports all kinds of yet unknown modalities, and we should try to proxy the upstream as faithfully as possible. So that even older versions of adapter-openai could be used as proxies for the bleeding edge OpenAI models...