fix: send responses only after state machine commit (#19286)

Fix a consistency issue where responses were sent to clients before
the state machine data was committed. Clients receiving early responses
could immediately read the state machine and observe stale or missing data.

Changes:
- Collect pending responses during entry application
- Send all responses only after `applier.commit()` succeeds
- Add doc comment explaining the consistency requirement

Author: drmingdrmer

src/meta/raft-store/src/sm_v003/sm_v003.rs

@@ -242,19 +242,31 @@ impl SMV003 {
         WriterAcquirer::new(self.write_semaphore.clone())
     }
 
+    /// Apply entries from the stream to the state machine.
+    ///
+    /// Responses are only sent AFTER commit succeeds to ensure consistency.
+    /// If we sent responses before commit, clients would consider their operations complete
+    /// and might immediately read the state machine, but the data wouldn't be persisted yet.
+    /// This could lead to clients observing stale or missing data.
     pub async fn apply_entries<S>(&self, mut entries: S) -> Result<(), io::Error>
     where S: Stream<Item = Result<EntryResponder, io::Error>> + Unpin {
         let mut applier = self.new_applier().await;
+        let mut pending_responses = Vec::new();
 
         while let Some(result) = entries.next().await {
             let (entry, responder) = result?;
             let applied = applier.apply(&entry).await?;
             if let Some(responder) = responder {
-                responder.send(applied);
+                pending_responses.push((responder, applied));
             }
         }
 
         applier.commit().await?;
+
+        for (responder, applied) in pending_responses {
+            responder.send(applied);
+        }
+
         Ok(())
     }