Preserve inherited previews for paginated thread forks (#60559)

## Why

Reference-backed forks can lack a local opening user message, causing them to disappear from thread listings or lose their inherited preview when a new turn is written. Compaction can also remove the opening message from the model context used to prepare a fork.

## What changed

- Persist inherited previews and preserve them when recording new turns and merging thread-list metadata.
- Include reference-backed forks with inherited history in thread listings before their first local message.
- Recover opening previews from transcript history within the requested fork boundary when compaction omits them from model context.
- Use the shared `user_message_preview` helper when deriving previews from rollout items.

## Testing

Extend thread-fork integration tests to check previews in listings before and after a new turn, after reopening, and when forking at a boundary after compaction.

GitOrigin-RevId: 5962cf71ea7104f07c9013cc77259b105e4c60d7

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -5702,12 +5702,19 @@ impl ThreadRequestProcessor {
             }
         }
 
-        let ephemeral_preview = if ephemeral {
-            if paginated_source && last_turn_id.is_none() && before_turn_id.is_none() {
-                source_thread.preview.clone()
-            } else {
-                preview_from_rollout_items(&history_items)
-            }
+        let inherited_preview = if paginated_source
+            && last_turn_id.is_none()
+            && before_turn_id.is_none()
+            && !source_thread.preview.is_empty()
+        {
+            source_thread.preview.clone()
+        } else if let Some(preview) = prepared_fork
+            .as_mut()
+            .and_then(|fork| fork.preview_before_compaction.take())
+        {
+            preview
+        } else if paginated_source || ephemeral {
+            preview_from_rollout_items(&history_items)
         } else {
             String::new()
         };
@@ -5727,6 +5734,7 @@ impl ThreadRequestProcessor {
                 self.thread_manager.as_ref(),
                 self.thread_store.as_ref(),
                 StoreThreadMetadataPatch {
+                    preview: (!inherited_preview.is_empty()).then(|| inherited_preview.clone()),
                     project_id: inherited_project_id.clone().map(Some),
                     daybreak_enabled: source_thread.daybreak_enabled,
                     ..Default::default()
@@ -5891,7 +5899,7 @@ impl ThreadRequestProcessor {
                 &config_snapshot,
                 /*path*/ None,
             );
-            thread.preview = ephemeral_preview;
+            thread.preview = inherited_preview;
             thread.forked_from_id = Some(source_thread_id.to_string());
             thread.turns = ephemeral_turns;
             (thread, ephemeral_token_usage_turn_id)
@@ -6912,12 +6920,15 @@ fn preview_from_rollout_items(items: &[RolloutItem]) -> String {
         .iter()
         .find_map(|item| match item {
             RolloutItem::ResponseItem(item) => match codex_core::parse_turn_item(&item.item) {
-                Some(codex_protocol::items::TurnItem::UserMessage(user)) => Some(user.message()),
+                Some(codex_protocol::items::TurnItem::UserMessage(user)) => {
+                    codex_protocol::protocol::user_message_preview(
+                        &user.as_legacy_user_message_event(),
+                    )
+                }
                 _ => None,
             },
             _ => None,
         })
-        .map(|preview| strip_user_message_prefix(preview.as_str()).to_string())
         .unwrap_or_default()
 }