Issues: #30994

I've implemented an important optimisation in response to GitHub
Copilot's recent rate limit on concurrent Vision API calls. Previously,
our system was defaulting to vision header: true for all API calls. To
prevent unnecessary calls and adhere to the new limits, I've updated our
logic: the vision header is now only sent if the current message is a
vision message, specifically when the preceding message includes an
image.

Prompt used to reproduce and verify the fix: `Give me a context for my
agent crate about. Browse my repo.`

Release Notes:

- copilot: Set Copilot-Vision-Request header based on message content