## Why
Cloud messages previously accepted only plain text, and switching a draft to Cloud waited for session preparation without processing composer input.
## What changed
- Support ordered text and image parts in Cloud submissions and user-message summaries, preserving attachment placeholders, image detail, and client message IDs.
- Accept inline data URLs and uploaded file IDs, including image-only messages. Reject invalid references and text spans locally, and cap encoded image requests at 3.75 MiB while retaining the existing plain-text limit.
- Keep the composer editable and redraw the TUI while Cloud prepares a conversation. Carry the latest draft, attachments, and Vim mode into the destination, retain drafts on failure, and prevent submission to the outgoing session during the handoff.
- Preserve buffered paste input and pending clipboard reads across the handoff, with connection and failure hints.
## Testing
Add regression coverage for image wire encoding, receipt identity, validation and size boundaries, draft editing during successful and failed handoffs, clipboard completion, Vim state, and attachment recovery after rejected submissions.
GitOrigin-RevId: 0cbcde0ab89ba961780b5f88456057e6b6a19cd5
Benjamin Carlsson
e6bf88ae02