fix(xai): map image_generation tool_choice to required

chat-proxy rejects allowed_tools lists that name image_generation.
Rewrite a forced image_generation choice, and an allowed_tools list
that only names that hosted tool, to the string "required". Mixed
allowed_tools lists drop the image_generation entry. Prune orphans
before this rewrite so older models still lose the leftover choice.

Closes: #5183