[archival service] fix get latest checkpoint request (#24435)

## Description 

fix get latest checkpoint request


---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] gRPC:
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] Indexing Framework:

Author: phoenix-o

crates/sui-kv-rpc/src/v2/get_checkpoint.rs

@@ -47,18 +47,11 @@ pub async fn get_checkpoint(
             .await?
             .pop()
             .ok_or(CheckpointNotFoundError::sequence_number(sequence_number))?,
-        None => {
-            let sequence_number = client.get_latest_checkpoint().await?;
-            client
-                .get_checkpoints(&[sequence_number])
-                .await?
-                .pop()
-                .ok_or(CheckpointNotFoundError::sequence_number(sequence_number))?
-        }
         _ => {
             let sequence_number = client.get_latest_checkpoint().await?;
+            let not_found_response = CheckpointNotFoundError::sequence_number(sequence_number);
             client
-                .get_checkpoints(&[sequence_number])
+                .get_checkpoints(&[sequence_number.checked_sub(1).ok_or(not_found_response)?])
                 .await?
                 .pop()
                 .ok_or(CheckpointNotFoundError::sequence_number(sequence_number))?