Move apply-patch file changes into turn items (#20540)

## Why

Apply-patch file changes are now part of the core turn item stream, so
v2 clients can consume the same first-class item lifecycle path used by
other turn items instead of relying on app-server-specific remapping
from legacy patch events.

## What changed

- Added a core `TurnItem::FileChange` carrying apply-patch changes and
completion metadata.
- Updated the apply-patch tool emitter to send `ItemStarted` /
`ItemCompleted` with the new `FileChange` item while preserving legacy
`PatchApplyBegin` / `PatchApplyEnd` fan-out.
- Updated app-server v2 conversion to render the new core item directly
and stopped `event_mapping` from remapping old patch begin/end events
into item notifications.
- Kept thread history reconstruction based on the existing old
apply-patch events for rollout compatibility.

## Verification

- `cargo test -p codex-protocol -p codex-app-server-protocol`
- `cargo test -p codex-core --test all
apply_patch_tool_executes_and_emits_patch_events`
- `cargo test -p codex-app-server bespoke_event_handling`

codex-rs/app-server-protocol/src/protocol/event_mapping.rs

@@ -1,7 +1,6 @@
 use crate::protocol::common::ServerNotification;
 use crate::protocol::item_builders::build_command_execution_begin_item;
 use crate::protocol::item_builders::build_command_execution_end_item;
-use crate::protocol::item_builders::build_file_change_begin_item;
 use crate::protocol::item_builders::convert_patch_changes;
 use crate::protocol::v2::AgentMessageDeltaNotification;
 use crate::protocol::v2::CollabAgentState;
@@ -450,13 +449,6 @@ pub fn item_event_to_server_notification(
                 item: item_completed_event.item.into(),
             })
         }
-        EventMsg::PatchApplyBegin(patch_begin_event) => {
-            ServerNotification::ItemStarted(ItemStartedNotification {
-                thread_id,
-                turn_id,
-                item: build_file_change_begin_item(&patch_begin_event),
-            })
-        }
         EventMsg::PatchApplyUpdated(event) => {
             ServerNotification::FileChangePatchUpdated(FileChangePatchUpdatedNotification {
                 thread_id,