Share stored JSON payloads across code mode cells with Arc (#47036)

## Why

Creating a cell deep-copies every stored JSON value, including values the cell never loads. Storing a value also duplicates its payload between the cell's local state and pending writes.

## What changed

Use `Arc<JsonValue>` throughout the session store, cell runtime, and completion writes to share immutable payloads. Keep a separate map snapshot for each cell so later commits cannot change its view of stored values.

GitOrigin-RevId: 4535f0f2ed0c8f8cbfb8d24cf1cb44cbc74472b5

codex-rs/code-mode-runtime/src/session_runtime/tests.rs

@@ -114,7 +114,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
     let commit = host.commit_completion(
         HashMap::from([(
             "candidate".to_string(),
-            JsonValue::String("lost".to_string()),
+            Arc::new(JsonValue::String("lost".to_string())),
         )]),
         completion.clone(),
         /*pending_initial_yield_items*/ None,