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/mod.rs

@@ -43,7 +43,9 @@ pub(crate) struct SessionRuntime {
 }
 
 struct Inner {
-    stored_values: Mutex<HashMap<String, JsonValue>>,
+    // Cells snapshot the keys while sharing immutable payloads, so later commits cannot
+    // change an existing cell's view and unused values do not need to be copied.
+    stored_values: Mutex<HashMap<String, Arc<JsonValue>>>,
     cells: Mutex<HashMap<CellId, CellHandle>>,
     cell_tasks: TaskTracker,
     shutdown_token: CancellationToken,
@@ -277,7 +279,7 @@ impl<D: SessionRuntimeDelegate> CellHost for RuntimeCellHost<D> {
 
     async fn commit_completion(
         &self,
-        stored_value_writes: HashMap<String, JsonValue>,
+        stored_value_writes: HashMap<String, Arc<JsonValue>>,
         event: CellEvent,
         pending_initial_yield_items: Option<Vec<OutputItem>>,
         cell_state: Arc<CellState>,

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,