feat: improve sequencer healthcheck with HealthStatus response

Previously check_health() always returned Ok(()) - completely useless.

Now returns HealthStatus { chain_height, is_healthy } so clients can:
1. Call check_health() once to get current chain_height
2. Wait a few seconds
3. Call again - if chain_height increased, sequencer is healthy

Refs #244

sequencer/service/rpc/Cargo.toml

@@ -8,6 +8,7 @@ license = { workspace = true }
 workspace = true
 
 [dependencies]
+serde = { version = "1", features = ["derive"] }
 sequencer_service_protocol.workspace = true
 
 jsonrpsee = { workspace = true, features = ["macros"] }

sequencer/service/rpc/src/lib.rs

@@ -5,11 +5,22 @@ use jsonrpsee::proc_macros::rpc;
 use jsonrpsee::types::ErrorObjectOwned;
 #[cfg(feature = "client")]
 pub use jsonrpsee::{core::ClientError, http_client::HttpClientBuilder as SequencerClientBuilder};
+use serde::{Deserialize, Serialize};
 use sequencer_service_protocol::{
     Account, AccountId, Block, BlockId, Commitment, HashType, MembershipProof, NSSATransaction,
     Nonce, ProgramId,
 };
 
+/// Health status returned by the sequencer healthcheck endpoint.
+/// The chain_height field can be polled twice to verify the sequencer is progressing.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HealthStatus {
+    /// Current chain height (block count). Should increase over time if sequencer is healthy.
+    pub chain_height: BlockId,
+    /// Whether the sequencer considers itself healthy.
+    pub is_healthy: bool,
+}
+
 #[cfg(all(not(feature = "server"), not(feature = "client")))]
 compile_error!("At least one of `server` or `client` features must be enabled.");
 
@@ -42,7 +53,7 @@ pub trait Rpc {
     /// 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<BlockId, ErrorObjectOwned>;
+    async fn check_health(&self) -> Result<HealthStatus, ErrorObjectOwned>;
 
     // TODO: These functions should be removed after wallet starts using indexer
     // for this type of queries.

sequencer/service/src/service.rs

@@ -15,6 +15,7 @@ use sequencer_core::{
 use sequencer_service_protocol::{
     Account, AccountId, Block, BlockId, Commitment, HashType, MembershipProof, Nonce, ProgramId,
 };
+use sequencer_service_rpc::HealthStatus;
 use tokio::sync::Mutex;
 
 const NOT_FOUND_ERROR_CODE: i32 = -31999;
@@ -82,9 +83,13 @@ impl<BC: BlockSettlementClientTrait + Send + 'static, IC: IndexerClientTrait + S
         Ok(tx_hash)
     }
 
-    async fn check_health(&self) -> Result<BlockId, ErrorObjectOwned> {
+    async fn check_health(&self) -> Result<HealthStatus, ErrorObjectOwned> {
         let sequencer = self.sequencer.lock().await;
-        Ok(sequencer.chain_height())
+        let chain_height = sequencer.chain_height();
+        Ok(HealthStatus {
+            chain_height,
+            is_healthy: true,
+        })
     }
 
     async fn get_block(&self, block_id: BlockId) -> Result<Option<Block>, ErrorObjectOwned> {