[codex] Emit image view as core item (#20512)

## Why

Image-view results should be represented as a core-produced turn item
instead of being reconstructed by app-server. At the same time, existing
rollout/history paths still understand the legacy `ViewImageToolCall`
event, so this keeps that event as compatibility output generated from
the new item lifecycle.

## What changed

- Added `TurnItem::ImageView` to `codex-protocol`.
- Emitted image-view item start/completion directly from the core
`view_image` handler.
- Kept `ViewImageToolCall` as a legacy event and generate it from
completed `TurnItem::ImageView` items.
- Kept `thread_history.rs` on the legacy `ViewImageToolCall` replay
path, with `ImageView` item lifecycle events ignored there.
- Updated app-server protocol conversion, rollout persistence, and
affected exhaustive event matches for the new item plus legacy fan-out
shape.

## Verification

- `cargo test -p codex-protocol -p codex-app-server-protocol -p
codex-rollout -p codex-rollout-trace -p codex-mcp-server -p
codex-app-server --lib`
- `cargo test -p codex-core --test all
view_image_tool_attaches_local_image`
- `just fix -p codex-protocol -p codex-core -p codex-app-server-protocol
-p codex-app-server -p codex-rollout -p codex-rollout-trace -p
codex-mcp-server`
- `git diff --check`

codex-rs/app-server/src/bespoke_event_handling.rs

@@ -954,28 +954,7 @@ pub(crate) async fn apply_bespoke_event_handling(
                 }))
                 .await;
         }
-        EventMsg::ViewImageToolCall(view_image_event) => {
-            let item = ThreadItem::ImageView {
-                id: view_image_event.call_id.clone(),
-                path: view_image_event.path.clone(),
-            };
-            let started = ItemStartedNotification {
-                thread_id: conversation_id.to_string(),
-                turn_id: event_turn_id.clone(),
-                item: item.clone(),
-            };
-            outgoing
-                .send_server_notification(ServerNotification::ItemStarted(started))
-                .await;
-            let completed = ItemCompletedNotification {
-                thread_id: conversation_id.to_string(),
-                turn_id: event_turn_id.clone(),
-                item,
-            };
-            outgoing
-                .send_server_notification(ServerNotification::ItemCompleted(completed))
-                .await;
-        }
+        EventMsg::ViewImageToolCall(_) => {}
         EventMsg::EnteredReviewMode(review_request) => {
             let review = review_request
                 .user_facing_hint