feat: improve sequencer healthcheck to return chain height

Previously check_health() always returned Ok(()) without checking
anything meaningful. A sequencer could be stuck and still pass.

Now check_health() returns the current chain height (BlockId).
Clients can call this twice with a delay to verify the chain is
progressing.

Refs #244

sequencer/service/rpc/src/lib.rs

@@ -39,9 +39,10 @@ pub trait Rpc {
     #[method(name = "sendTransaction")]
     async fn send_transaction(&self, tx: NSSATransaction) -> Result<HashType, ErrorObjectOwned>;
 
-    // TODO: expand healthcheck response into some kind of report
+    /// Returns the current chain height (block count).
+    /// A sequencer is healthy if this value increases over time.
     #[method(name = "checkHealth")]
-    async fn check_health(&self) -> Result<(), ErrorObjectOwned>;
+    async fn check_health(&self) -> Result<BlockId, ErrorObjectOwned>;
 
     // TODO: These functions should be removed after wallet starts using indexer
     // for this type of queries.

sequencer/service/src/service.rs

@@ -82,8 +82,9 @@ impl<BC: BlockSettlementClientTrait + Send + 'static, IC: IndexerClientTrait + S
         Ok(tx_hash)
     }
 
-    async fn check_health(&self) -> Result<(), ErrorObjectOwned> {
-        Ok(())
+    async fn check_health(&self) -> Result<BlockId, ErrorObjectOwned> {
+        let sequencer = self.sequencer.lock().await;
+        Ok(sequencer.chain_height())
     }
 
     async fn get_block(&self, block_id: BlockId) -> Result<Option<Block>, ErrorObjectOwned> {