Part of #1592

Scope

Wire up image input via Gemini's inlineData parts format.

Files to Modify

• crates/zeph-llm/src/gemini.rs -- convert MessagePart::Image to inlineData part in message builder

Key Implementation Details

• Images sent as inlineData parts within contents[].parts[]:

  { "inlineData": { "mimeType": "image/jpeg", "data": "base64..." } }

• All Gemini 2.0+ models support vision natively (no separate vision model needed)
• Multiple images per message supported
• Mixed text + image parts in single message supported
• Zeph's ImageData { data: Vec<u8>, mime_type: String } maps directly

Acceptance Criteria

• supports_vision() returns true (already set in Phase 1)
• MessagePart::Image correctly converted to inlineData format
• Multiple images in single message work
• Mixed text + image parts produce correct parts array
• Unit tests with mock image payloads