Persist thread section moves before the first turn (#39523)

## Why

New non-ephemeral threads have no persisted rollout or preview until their first
turn, so moving them into a section could leave them absent from section-filtered
thread lists.

## What changed

- Materialize and flush a non-ephemeral thread before applying an explicit
  section move.
- Include explicitly sectioned threads with empty previews in section-filtered
  queries, and update the partial indexes to cover them.

## Testing

- Add an app-server test that moves a newly started thread into and out of the
  pinned section before its first turn.
- Cover empty-preview threads in recency and section-position query tests.

GitOrigin-RevId: aa3c4daf9890f06c08b93cd60f146099cbc6cfb2

codex-rs/core/src/thread_manager.rs

@@ -835,19 +835,22 @@ impl ThreadManager {
         }
     }
 
-    /// Moves a persisted thread to, within, or out of a server-ordered section.
+    /// Moves a thread to, within, or out of a server-ordered section.
     pub async fn move_thread_to_section(
         &self,
         thread_id: ThreadId,
         section: Option<&str>,
         before_thread_id: Option<ThreadId>,
     ) -> CodexResult<()> {
-        if let Ok(thread) = self.get_thread(thread_id).await
-            && thread.config_snapshot().await.ephemeral
-        {
-            return Err(CodexErr::InvalidRequest(format!(
-                "ephemeral thread does not support section moves: {thread_id}"
-            )));
+        if let Ok(thread) = self.get_thread(thread_id).await {
+            if thread.config_snapshot().await.ephemeral {
+                return Err(CodexErr::InvalidRequest(format!(
+                    "ephemeral thread does not support section moves: {thread_id}"
+                )));
+            }
+            // Explicit placement must work before the first turn materializes the thread.
+            thread.ensure_rollout_materialized().await;
+            thread.flush_rollout().await?;
         }
 
         self.state