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>

common/warp_utils/src/reject.rs

@@ -110,6 +110,17 @@ pub fn not_synced(msg: String) -> warp::reject::Rejection {
     warp::reject::custom(NotSynced(msg))
 }
 
+/// A 404 Not Found response for when no block has been received for the
+/// requested slot.
+#[derive(Debug)]
+pub struct BlockNotFound(pub String);
+
+impl Reject for BlockNotFound {}
+
+pub fn block_not_found(msg: String) -> warp::reject::Rejection {
+    warp::reject::custom(BlockNotFound(msg))
+}
+
 #[derive(Debug)]
 pub struct InvalidAuthorization(pub String);
 
@@ -199,6 +210,9 @@ pub async fn handle_rejection(err: warp::Rejection) -> Result<impl warp::Reply,
     } else if let Some(e) = err.find::<crate::reject::NotSynced>() {
         code = StatusCode::SERVICE_UNAVAILABLE;
         message = format!("SERVICE_UNAVAILABLE: beacon node is syncing: {}", e.0);
+    } else if let Some(e) = err.find::<crate::reject::BlockNotFound>() {
+        code = StatusCode::NOT_FOUND;
+        message = format!("NOT_FOUND: {}", e.0);
     } else if let Some(e) = err.find::<crate::reject::InvalidAuthorization>() {
         code = StatusCode::FORBIDDEN;
         message = format!("FORBIDDEN: Invalid auth token: {}", e.0);