fix(en): Return SyncState health check (#3142)

## What ❔

Returns `SyncState`-based health check, which was removed when
transitioning to the node framework.

## Why ❔

This health check looks useful and is used at least in some internal
automations.

## Checklist

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [x] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zkstack dev fmt` and `zkstack dev
lint`.

core/bin/external_node/src/tests/mod.rs

@@ -22,10 +22,18 @@ const POLL_INTERVAL: Duration = Duration::from_millis(100);
 #[tracing::instrument] // Add args to the test logs
 async fn external_node_basics(components_str: &'static str) {
     let _guard = zksync_vlog::ObservabilityBuilder::new().try_build().ok(); // Enable logging to simplify debugging
-
     let (env, env_handles) = utils::TestEnvironment::with_genesis_block(components_str).await;
 
-    let expected_health_components = utils::expected_health_components(&env.components);
+    let mut expected_health_components = utils::expected_health_components(&env.components);
+    let expected_shutdown_components = expected_health_components.clone();
+    let has_core_or_api = env.components.0.iter().any(|component| {
+        [Component::Core, Component::HttpApi, Component::WsApi].contains(component)
+    });
+    if has_core_or_api {
+        // The `sync_state` component doesn't signal its shutdown, but should be present in the list of components
+        expected_health_components.push("sync_state");
+    }
+
     let l2_client = utils::mock_l2_client(&env);
     let eth_client = utils::mock_eth_client(env.config.diamond_proxy_address());
 
@@ -84,7 +92,7 @@ async fn external_node_basics(components_str: &'static str) {
     let health_data = app_health.check_health().await;
     tracing::info!(?health_data, "final health data");
     assert_matches!(health_data.inner().status(), HealthStatus::ShutDown);
-    for name in expected_health_components {
+    for name in expected_shutdown_components {
         let component_health = &health_data.components()[name];
         assert_matches!(component_health.status(), HealthStatus::ShutDown);
     }

core/node/node_framework/src/implementations/layers/state_keeper/external_io.rs

@@ -8,6 +8,7 @@ use zksync_types::L2ChainId;
 use crate::{
     implementations::resources::{
         action_queue::ActionQueueSenderResource,
+        healthcheck::AppHealthCheckResource,
         main_node_client::MainNodeClientResource,
         pools::{MasterPool, PoolResource},
         state_keeper::{ConditionalSealerResource, StateKeeperIOResource},
@@ -26,6 +27,7 @@ pub struct ExternalIOLayer {
 #[derive(Debug, FromContext)]
 #[context(crate = crate)]
 pub struct Input {
+    pub app_health: AppHealthCheckResource,
     pub pool: PoolResource<MasterPool>,
     pub main_node_client: MainNodeClientResource,
 }
@@ -57,6 +59,10 @@ impl WiringLayer for ExternalIOLayer {
     async fn wire(self, input: Self::Input) -> Result<Self::Output, WiringError> {
         // Create `SyncState` resource.
         let sync_state = SyncState::default();
+        let app_health = &input.app_health.0;
+        app_health
+            .insert_custom_component(Arc::new(sync_state.clone()))
+            .map_err(WiringError::internal)?;
 
         // Create `ActionQueueSender` resource.
         let (action_queue_sender, action_queue) = ActionQueue::new();

core/node/node_framework/src/implementations/layers/sync_state_updater.rs

@@ -1,9 +1,12 @@
+use std::sync::Arc;
+
 use zksync_dal::{ConnectionPool, Core};
 use zksync_node_sync::SyncState;
 use zksync_web3_decl::client::{DynClient, L2};
 
 use crate::{
     implementations::resources::{
+        healthcheck::AppHealthCheckResource,
         main_node_client::MainNodeClientResource,
         pools::{MasterPool, PoolResource},
         sync_state::SyncStateResource,
@@ -24,6 +27,7 @@ pub struct SyncStateUpdaterLayer;
 pub struct Input {
     /// Fetched to check whether the `SyncState` was already provided by another layer.
     pub sync_state: Option<SyncStateResource>,
+    pub app_health: AppHealthCheckResource,
     pub master_pool: PoolResource<MasterPool>,
     pub main_node_client: MainNodeClientResource,
 }
@@ -62,6 +66,10 @@ impl WiringLayer for SyncStateUpdaterLayer {
         let MainNodeClientResource(main_node_client) = input.main_node_client;
 
         let sync_state = SyncState::default();
+        let app_health = &input.app_health.0;
+        app_health
+            .insert_custom_component(Arc::new(sync_state.clone()))
+            .map_err(WiringError::internal)?;
 
         Ok(Output {
             sync_state: Some(sync_state.clone().into()),

core/node/node_sync/src/sync_state.rs

@@ -173,6 +173,7 @@ impl CheckHealth for SyncState {
         Health::from(&*self.0.borrow())
     }
 }
+
 impl SyncStateInner {
     fn is_synced(&self) -> (bool, Option<u32>) {
         if let (Some(main_node_block), Some(local_block)) = (self.main_node_block, self.local_block)