Make turn diff tracking operation backed (#21180)

## Summary
- replace filesystem-based turn diff tracking with an operation-backed
accumulator
- preserve enough verified apply_patch state to render move-overwrite
cases correctly
- keep the turn/diff/updated contract intact while removing remote-only
turn-diff test skips

This takes the assumption that no 3P services rely on the output format
of `apply_patch`

## Why
For the CCA file system isolation push

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/unified_exec/async_watcher.rs

@@ -226,7 +226,13 @@ pub(crate) async fn emit_exec_end_for_unified_exec(
         process_id,
     );
     emitter
-        .emit(event_ctx, ToolEventStage::Success(output))
+        .emit(
+            event_ctx,
+            ToolEventStage::Success {
+                output,
+                applied_patch_delta: None,
+            },
+        )
         .await;
 }