Fix annotation schema to accept "file" type for file uploads

[HashimMufti]

Problem

The annotation validation schema only accepts type: "url_citation", but OpenRouter API returns
type: "file" when processing file uploads (PDFs, images, etc.) with vision models. This
causes validation errors that prevent users from using file processing features.

Error

AI_TypeValidationError: Invalid input: expected "url_citation"
path: ["choices", 0, "message", "annotations", 0, "type"]

Root Cause

When OpenRouter processes files with models like qwen/qwen3-235b-a22b-2507 or other
vision models using the pdf-text or mistral-ocr engines, it returns file annotations as a
cost optimization feature (see OpenRouter docs on skipping parsing
costs).

The current schema in src/chat/schemas.ts only validates type: "url_citation", causing the
validation to fail when type: "file" is returned.

[HashimMufti]

Fixed by #208, closed.
Cheers @subtleGradient!