chore: sanbox extraction (#4286)

# Extract and Centralize Sandboxing
- Goal: Improve safety and clarity by centralizing sandbox planning and
execution.
  - Approach:
- Add planner (ExecPlan) and backend registry (Direct/Seatbelt/Linux)
with run_with_plan.
- Refactor codex.rs to plan-then-execute; handle failures/escalation via
the plan.
- Delegate apply_patch to the codex binary and run it with an empty env
for determinism.

codex-rs/core/src/apply_patch.rs

@@ -27,6 +27,7 @@ pub(crate) enum InternalApplyPatchInvocation {
     DelegateToExec(ApplyPatchExec),
 }
 
+#[derive(Debug)]
 pub(crate) struct ApplyPatchExec {
     pub(crate) action: ApplyPatchAction,
     pub(crate) user_explicitly_approved_this_action: bool,
@@ -109,3 +110,28 @@ pub(crate) fn convert_apply_patch_to_protocol(
     }
     result
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    use tempfile::tempdir;
+
+    #[test]
+    fn convert_apply_patch_maps_add_variant() {
+        let tmp = tempdir().expect("tmp");
+        let p = tmp.path().join("a.txt");
+        // Create an action with a single Add change
+        let action = ApplyPatchAction::new_add_for_test(&p, "hello".to_string());
+
+        let got = convert_apply_patch_to_protocol(&action);
+
+        assert_eq!(
+            got.get(&p),
+            Some(&FileChange::Add {
+                content: "hello".to_string()
+            })
+        );
+    }
+}