PDF inputs

PDFs are sent as base64 data with an optional parsing-engine hint (text vs. OCR). File annotations from a first request can be reused on follow-ups to avoid re-parsing cost.

See examples/pdf-inputs/main.go for:

• Basic PDF upload
• Choosing an engine with WithPDFEngine
• Reusing annotations across requests
• Base64 vs. URL inputs
• Engine comparison

Models must support document inputs.