[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/core/src/session/turn.rs

@@ -1470,9 +1470,9 @@ pub(super) fn realtime_text_for_event(msg: &EventMsg) -> Option<String> {
         | EventMsg::PatchApplyBegin(_)
         | EventMsg::PatchApplyUpdated(_)
         | EventMsg::PatchApplyEnd(_)
-        | EventMsg::ViewImageToolCall(_)
         | EventMsg::ImageGenerationBegin(_)
         | EventMsg::ImageGenerationEnd(_)
+        | EventMsg::ViewImageToolCall(_)
         | EventMsg::ExecApprovalRequest(_)
         | EventMsg::RequestPermissions(_)
         | EventMsg::RequestUserInput(_)