code-mode: merge stored values by key (#24159)

## Summary

Change code-mode stored value updates to merge writes by key instead of
replacing the session's complete stored-value map after each cell
completes.

Previously, each cell received a snapshot of stored values and returned
the complete resulting map. When multiple cells ran concurrently, a
later completion could overwrite values written by another cell because
it committed an older snapshot.

This change moves stored-value ownership into `CodeModeService`:

- Each runtime starts from the service's current stored values.
- Runtime completion reports only keys written by that cell.
- The service merges those writes into the current stored-value map on
successful completion.
- Core no longer replaces its stored-value state from a cell result.

As a result, concurrently executing cells can update different stored
keys without clobbering one another.

The move into CodeModeService is motivated by a desire to have this
lifetime tied to a new lifetime object on that side in a subsequent PR.

codex-rs/core/src/tools/code_mode/execute_handler.rs

@@ -38,12 +38,6 @@ impl CodeModeExecuteHandler {
         let exec = ExecContext { session, turn };
         let enabled_tools =
             codex_tools::collect_code_mode_tool_definitions(&self.nested_tool_specs);
-        let stored_values = exec
-            .session
-            .services
-            .code_mode_service
-            .stored_values()
-            .await;
         // Allocate before starting V8 so the trace can create the parent
         // CodeCell before model-authored JavaScript issues nested tool calls.
         let runtime_cell_id = exec.session.services.code_mode_service.allocate_cell_id();
@@ -67,7 +61,6 @@ impl CodeModeExecuteHandler {
                 tool_call_id: call_id,
                 enabled_tools,
                 source: args.code,
-                stored_values,
                 yield_time_ms: args.yield_time_ms,
                 max_output_tokens: args.max_output_tokens,
             })

codex-rs/core/src/tools/code_mode/mod.rs

@@ -66,17 +66,6 @@ impl CodeModeService {
         }
     }
 
-    pub(crate) async fn stored_values(&self) -> std::collections::HashMap<String, JsonValue> {
-        self.inner.stored_values().await
-    }
-
-    pub(crate) async fn replace_stored_values(
-        &self,
-        values: std::collections::HashMap<String, JsonValue>,
-    ) {
-        self.inner.replace_stored_values(values).await;
-    }
-
     pub(crate) fn allocate_cell_id(&self) -> String {
         self.inner.allocate_cell_id()
     }
@@ -182,17 +171,11 @@ pub(super) async fn handle_runtime_response(
         }
         RuntimeResponse::Result {
             content_items,
-            stored_values,
             error_text,
             ..
         } => {
             let mut content_items = into_function_call_output_content_items(content_items);
             sanitize_runtime_image_detail(exec.turn.as_ref(), &mut content_items);
-            exec.session
-                .services
-                .code_mode_service
-                .replace_stored_values(stored_values)
-                .await;
             let success = error_text.is_none();
             if let Some(error_text) = error_text {
                 content_items.push(FunctionCallOutputContentItem::InputText {