Add Warning for Downloaded Assets

[BrianHung]

Feature Description

https://github.com/vercel/ai/blob/main/packages/ai/core/prompt/convert-to-language-model-prompt.ts

Depending on the LLM, image and file are assets are downloaded and inlined into the messages fed to the model. This can lead to unexpected memory errors if the image and files are quite large. It is also a leaky abstraction if you give urls inside of messages, and they are automatically inlined as the expectation is urls are maintained.

At the minimum, I think there should be console warns that a model doesn't support URLs directly and assets have to be inlined.

Use Cases

No response

Additional context

No response