Use captured step settings when building model context (#44200)

## Why

Model changes during an active turn could leave context using the turn's initial model and settings, even after requests switched to the new model.

## What changed

- Build model, personality, approval, collaboration, app, plugin, and multi-agent instructions from captured step settings.
- Resolve multi-agent policy from the step's effective reasoning effort.
- Use the captured step's context window for full-context metadata and retain that step when rebuilding initial context for compaction.

## Testing

Add regression coverage for model switches updating context without duplicating updates, delegation policy following selected or model-default reasoning effort, and compaction preserving retained context after a model change.

GitOrigin-RevId: 3615c7a3f13f15fdcce9ad13d23311340e0aea88

codex-rs/core/src/compact_tests.rs

@@ -26,7 +26,7 @@ async fn process_compacted_history_with_test_session(
             .expect("world state should build"),
     );
     let initial_context = session
-        .build_initial_context_with_world_state(&turn_context, world_state.as_ref())
+        .build_initial_context_with_world_state(&step_context, world_state.as_ref())
         .await;
     let initial_context_injection = InitialContextInjection::BeforeLastUserMessage {
         world_state,