## Why
Remote image submissions currently wrap native `input_image` spans in
literal `<image>` and `</image>` text spans. Those extra prompt tokens
add structure without providing label or routing information.
## What Changed
- Serialize `UserInput::Image` directly as an `input_image` content
span.
- Preserve named local-image framing and legacy wrapper parsing for
labeled attachments and existing histories.
- Update existing request-shape expectations for drag-and-drop images,
model switching, and compaction.
## Validation
- `just test -p codex-protocol`
- Focused `codex-core` run covering
`drag_drop_image_persists_rollout_request_shape`,
`model_change_from_image_to_text_strips_prior_image_content`, and
`snapshot_request_shape_pre_turn_compaction_including_incoming_user_message`
## Notes
- A broader `just test -p codex-core` run was attempted; the affected
tests passed, while the overall run failed in unrelated CLI, MCP, and
tooling tests plus a `thread_manager` timeout.
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