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/code-mode/src/runtime/callbacks.rs

@@ -157,7 +157,8 @@ pub(super) fn store_callback(
         }
     };
     if let Some(state) = scope.get_slot_mut::<RuntimeState>() {
-        state.stored_values.insert(key, serialized);
+        state.stored_values.insert(key.clone(), serialized.clone());
+        state.stored_value_writes.insert(key, serialized);
     }
 }