Add step-scoped data to extension contributors (#34525)

## What changed

- Add an `ExtensionData` store to each `StepContext`.
- Pass the step store to context, world-state, turn-input, and tool contributors so extensions can use capabilities bound to the current sampling step.
- Preserve the captured step context when rebuilding initial context during compaction.

GitOrigin-RevId: 51f3105d3a793f60534f122f9be8247c131c2ca5

codex-rs/ext/goal/src/extension.rs

@@ -420,6 +420,7 @@ where
         &self,
         _session_store: &ExtensionData,
         thread_store: &ExtensionData,
+        _step_store: &ExtensionData,
     ) -> Vec<Arc<dyn codex_extension_api::ToolExecutor<codex_extension_api::ToolCall>>> {
         let Some(runtime) = goal_runtime_handle(thread_store) else {
             return Vec::new();