rework message styling (#2877)

https://github.com/user-attachments/assets/cf07f62b-1895-44bb-b9c3-7a12032eb371

codex-rs/core/src/turn_diff_tracker.rs

@@ -578,7 +578,12 @@ index {ZERO_OID}..{right_oid}
         fs::write(&file, "x\n").unwrap();
 
         let mut acc = TurnDiffTracker::new();
-        let del_changes = HashMap::from([(file.clone(), FileChange::Delete)]);
+        let del_changes = HashMap::from([(
+            file.clone(),
+            FileChange::Delete {
+                content: "x\n".to_string(),
+            },
+        )]);
         acc.on_patch_begin(&del_changes);
 
         // Simulate apply: delete the file from disk.
@@ -741,7 +746,12 @@ index {left_oid}..{right_oid}
         assert_eq!(first, expected_first);
 
         // Next: introduce a brand-new path b.txt into baseline snapshots via a delete change.
-        let del_b = HashMap::from([(b.clone(), FileChange::Delete)]);
+        let del_b = HashMap::from([(
+            b.clone(),
+            FileChange::Delete {
+                content: "z\n".to_string(),
+            },
+        )]);
         acc.on_patch_begin(&del_b);
         // Simulate apply: delete b.txt.
         let baseline_mode = file_mode_for_path(&b).unwrap_or(FileMode::Regular);