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/module_loader.rs

@@ -104,14 +104,14 @@ pub(super) fn completion_state(
     scope: &mut v8::PinScope<'_, '_>,
     pending_promise: Option<&v8::Global<v8::Promise>>,
 ) -> CompletionState {
-    let stored_values = scope
+    let stored_value_writes = scope
         .get_slot::<RuntimeState>()
-        .map(|state| state.stored_values.clone())
+        .map(|state| state.stored_value_writes.clone())
         .unwrap_or_default();
 
     let Some(pending_promise) = pending_promise else {
         return CompletionState::Completed {
-            stored_values,
+            stored_value_writes,
             error_text: None,
         };
     };
@@ -120,7 +120,7 @@ pub(super) fn completion_state(
     match promise.state() {
         v8::PromiseState::Pending => CompletionState::Pending,
         v8::PromiseState::Fulfilled => CompletionState::Completed {
-            stored_values,
+            stored_value_writes,
             error_text: None,
         },
         v8::PromiseState::Rejected => {
@@ -131,7 +131,7 @@ pub(super) fn completion_state(
                 Some(value_to_error_text(scope, result))
             };
             CompletionState::Completed {
-                stored_values,
+                stored_value_writes,
                 error_text,
             }
         }