Include context window IDs in response metadata (#39847)

## What changed

- Add `context_window_id` to turn metadata for sampling and compaction requests, using the UUID for the model-visible context window.
- Keep the ID stable for requests in the same context window and advance it after compaction, independently of the existing thread-generation `window_id`.
- Reserve `context_window_id` so client metadata cannot override it.

## Testing

- Cover HTTP and WebSocket metadata, remote compaction, token-budget context alignment, resume behavior, and post-compaction window changes.

GitOrigin-RevId: 55cceea377ad509c8dd96ad668f2b5703ea46a63

codex-rs/core/src/session/mod.rs

@@ -3769,10 +3769,15 @@ impl Session {
     }
 
     pub(crate) async fn current_window_id(&self) -> String {
+        self.current_window().await.0
+    }
+
+    pub(crate) async fn current_window(&self) -> (String, Uuid) {
         let state = self.state.lock().await;
         let thread_id = self.thread_id;
         let window_number = state.auto_compact_window_number();
-        format!("{thread_id}:{window_number}")
+        let context_window_id = state.auto_compact_window_ids().window_id;
+        (format!("{thread_id}:{window_number}"), context_window_id)
     }
 
     pub(crate) async fn advance_auto_compact_window(&self) -> (u64, AutoCompactWindowIds) {