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/thread_history.rs

@@ -357,6 +357,7 @@ impl ThreadHistoryBuilder {
             | codex_protocol::items::TurnItem::Reasoning(_)
             | codex_protocol::items::TurnItem::WebSearch(_)
             | codex_protocol::items::TurnItem::ImageGeneration(_)
+            | codex_protocol::items::TurnItem::FileChange(_)
             | codex_protocol::items::TurnItem::ContextCompaction(_) => {}
         }
     }
@@ -378,6 +379,7 @@ impl ThreadHistoryBuilder {
             | codex_protocol::items::TurnItem::Reasoning(_)
             | codex_protocol::items::TurnItem::WebSearch(_)
             | codex_protocol::items::TurnItem::ImageGeneration(_)
+            | codex_protocol::items::TurnItem::FileChange(_)
             | codex_protocol::items::TurnItem::ContextCompaction(_) => {}
         }
     }