[CRITICAL] No ImageContent/DocumentContent in InputContentBlock

[tuterx]

InputContentBlock only has Text/Thinking/ToolUse/ToolResult. Missing Image and Document variants. Users cannot paste screenshots, upload PDFs, or attach files.

Evidence: rust/crates/api/src/types.rs InputContentBlock enum — 4 variants, no Image/Document.

Fix: Add ImageContent { source: ImageSource } + DocumentContent { source: DocumentSource } variants. Support base64 and URL sources.

[Yeachan-Heo]

Tracked in ROADMAP.md as an architectural gap. This is a known limitation, not a bug.