Implement proposer lookahead endpoint (#8815)

closes #8809


  Implement GET /eth/v1/beacon/states/{state_id}/proposer_lookahead ([beacon-APIs#565](https://github.com/ethereum/beacon-APIs/pull/565)). Returns the proposer lookahead from Fulu+ states; 400 for pre-Fulu. Includes integration test.


Co-Authored-By: romeoscript <romeobourne211@gmail.com>

Co-Authored-By: Tan Chee Keong <tanck@sigmaprime.io>

beacon_node/http_api/tests/tests.rs

@@ -37,7 +37,7 @@ use proto_array::ExecutionStatus;
 use reqwest::{RequestBuilder, Response, StatusCode};
 use sensitive_url::SensitiveUrl;
 use slot_clock::SlotClock;
-use ssz::BitList;
+use ssz::{BitList, Decode};
 use state_processing::per_block_processing::get_expected_withdrawals;
 use state_processing::per_slot_processing;
 use state_processing::state_advance::partial_state_advance;
@@ -1409,6 +1409,72 @@ impl ApiTester {
         self
     }
 
+    pub async fn test_beacon_states_proposer_lookahead(self) -> Self {
+        for state_id in self.interesting_state_ids() {
+            let mut state_opt = state_id
+                .state(&self.chain)
+                .ok()
+                .map(|(state, _execution_optimistic, _finalized)| state);
+
+            let result = match self
+                .client
+                .get_beacon_states_proposer_lookahead(state_id.0)
+                .await
+            {
+                Ok(response) => response,
+                Err(e) => panic!("query failed incorrectly: {e:?}"),
+            };
+
+            if result.is_none() && state_opt.is_none() {
+                continue;
+            }
+
+            let state = state_opt.as_mut().expect("result should be none");
+            let expected = state.proposer_lookahead().unwrap();
+
+            let response = result.unwrap();
+            assert_eq!(response.data(), &expected.to_vec());
+
+            // Check that the version header is returned in the response
+            let fork_name = state.fork_name(&self.chain.spec).unwrap();
+            assert_eq!(response.version(), Some(fork_name),);
+        }
+
+        self
+    }
+
+    pub async fn test_beacon_states_proposer_lookahead_ssz(self) -> Self {
+        for state_id in self.interesting_state_ids() {
+            let mut state_opt = state_id
+                .state(&self.chain)
+                .ok()
+                .map(|(state, _execution_optimistic, _finalized)| state);
+
+            let result = match self
+                .client
+                .get_beacon_states_proposer_lookahead_ssz(state_id.0)
+                .await
+            {
+                Ok(response) => response,
+                Err(e) => panic!("query failed incorrectly: {e:?}"),
+            };
+
+            if result.is_none() && state_opt.is_none() {
+                continue;
+            }
+
+            let state = state_opt.as_mut().expect("result should be none");
+            let expected = state.proposer_lookahead().unwrap();
+
+            let ssz_bytes = result.unwrap();
+            let decoded = Vec::<u64>::from_ssz_bytes(&ssz_bytes)
+                .expect("should decode SSZ proposer lookahead");
+            assert_eq!(decoded, expected.to_vec());
+        }
+
+        self
+    }
+
     pub async fn test_beacon_headers_all_slots(self) -> Self {
         for slot in 0..CHAIN_LENGTH {
             let slot = Slot::from(slot);
@@ -7360,6 +7426,23 @@ async fn beacon_get_state_info_electra() {
         .await;
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn beacon_get_state_info_fulu() {
+    let mut config = ApiTesterConfig::default();
+    config.spec.altair_fork_epoch = Some(Epoch::new(0));
+    config.spec.bellatrix_fork_epoch = Some(Epoch::new(0));
+    config.spec.capella_fork_epoch = Some(Epoch::new(0));
+    config.spec.deneb_fork_epoch = Some(Epoch::new(0));
+    config.spec.electra_fork_epoch = Some(Epoch::new(0));
+    config.spec.fulu_fork_epoch = Some(Epoch::new(0));
+    ApiTester::new_from_config(config)
+        .await
+        .test_beacon_states_proposer_lookahead()
+        .await
+        .test_beacon_states_proposer_lookahead_ssz()
+        .await;
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn beacon_get_blocks() {
     ApiTester::new()