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/cell_actor/tests.rs

@@ -43,7 +43,7 @@ impl CellHost for TestHost {
 
     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>,
@@ -75,7 +75,7 @@ impl CellHost for RecordingHost {
 
     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>,