fix: payload_attestation_data when no block received for slot (#9225)

Addresses issue #9220

The `payload_attestation_data` endpoint returns 400 when no block has been received for the requested slot. This causes the VC to log at CRIT level for what is expected behaviour per spec: validators should simply not submit a payload attestation when no block has been seen.


  - Return 404 (Not Found) instead of 400 from `payload_attestation_data` when no block exists for the slot. This is consistent with other beacon api endpoints.
- Downgrade the VC log from `crit` to `debug` when a 503 is received, since this is an expected no-op per spec.
- Add `BlockNotFound` rejection type to `warp_utils`.
- Add a test asserting the 404 response for an empty slot.


Co-Authored-By: Josh King <josh@sigmaprime.io>

Co-Authored-By: Eitan Seri-Levi <eserilev@ucsc.edu>

Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

beacon_node/http_api/tests/tests.rs

@@ -4807,7 +4807,8 @@ impl ApiTester {
             .client
             .get_validator_payload_attestation_data(slot)
             .await
-            .unwrap();
+            .unwrap()
+            .expect("expected payload attestation data for slot with block");
 
         assert_eq!(response.version(), Some(fork_name));
 
@@ -4823,7 +4824,8 @@ impl ApiTester {
             .client
             .get_validator_payload_attestation_data_ssz(slot)
             .await
-            .unwrap();
+            .unwrap()
+            .expect("expected SSZ payload attestation data for slot with block");
 
         assert_eq!(ssz_result, expected);
 
@@ -4894,6 +4896,7 @@ impl ApiTester {
                 .get_validator_payload_attestation_data(slot)
                 .await
                 .unwrap()
+                .expect("expected payload attestation data for slot with block")
                 .into_data();
 
             assert_eq!(pa_data.beacon_block_root, block_root);
@@ -4926,6 +4929,26 @@ impl ApiTester {
         self
     }
 
+    pub async fn test_get_validator_payload_attestation_data_no_block(self) -> Self {
+        // Advance the slot clock without producing a block
+        self.harness.advance_slot();
+        let slot = self.chain.slot().unwrap();
+
+        // Should return None when no block exists for the slot
+        let result = self
+            .client
+            .get_validator_payload_attestation_data(slot)
+            .await
+            .unwrap();
+
+        assert!(
+            result.is_none(),
+            "expected None for empty slot, got: {result:?}"
+        );
+
+        self
+    }
+
     #[allow(clippy::await_holding_lock)] // This is a test, so it should be fine.
     pub async fn test_get_validator_aggregate_attestation_v1(self) -> Self {
         let attestation = self
@@ -8597,6 +8620,17 @@ async fn get_validator_payload_attestation_data_pre_gloas() {
         .await;
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn get_validator_payload_attestation_data_no_block() {
+    if !fork_name_from_env().is_some_and(|f| f.gloas_enabled()) {
+        return;
+    }
+    ApiTester::new_with_hard_forks()
+        .await
+        .test_get_validator_payload_attestation_data_no_block()
+        .await;
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn payload_attestation_present_after_envelope_publish() {
     ApiTester::new_with_hard_forks()