[codex] remove plain image wrapper spans

[pakrym-oai]

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.