Support window prefixes for extension world-state sections (#57276)

## What changed

Add `WorldStateSectionContribution::with_window_prefix()` to render extension developer fragments as separate messages before retained history when building a context window, including after compaction. Updates within an existing window still append to history. Ignore fragments with other roles for sections that opt in.

## Testing

Add a unit test for prefix placement and duplicate avoidance, plus local and remote compaction scenarios covering live updates, persisted replacement history, and rejection of non-developer fragments.

GitOrigin-RevId: 8621457c6eec17f1585ca8e6fa154b1bf3257330

codex-rs/core/src/context/world_state/mod.rs

@@ -247,11 +247,20 @@ impl ErasedWorldStateSection for ExtensionWorldStateSection {
         });
         let updates = fragment
             .into_iter()
+            .filter(|fragment| !self.0.is_window_prefix() || fragment.role() == "developer")
             .map(|fragment| {
-                WorldStateUpdate::fragment(WorldStateContextFragment {
+                let update = WorldStateUpdate::fragment(WorldStateContextFragment {
                     fragment,
                     content_kind: ContentItemKind(format!("{}.instructions", self.0.id())),
-                })
+                });
+                if self.0.is_window_prefix() {
+                    WorldStateUpdate {
+                        placement: Placement::Prefix,
+                        ..update
+                    }
+                } else {
+                    update
+                }
             })
             .collect();
         (snapshot, updates)