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/src/validator/mod.rs

@@ -333,8 +333,12 @@ pub fn get_validator_payload_attestation_data<T: BeaconChainTypes>(
                     let payload_attestation_data = chain
                         .produce_payload_attestation_data(slot)
                         .map_err(|e| match e {
-                            BeaconChainError::InvalidSlot(_)
-                            | BeaconChainError::NoBlockForSlot(_) => {
+                            BeaconChainError::NoBlockForSlot(_) => {
+                                warp_utils::reject::block_not_found(format!(
+                                    "No block received for slot {slot}"
+                                ))
+                            }
+                            BeaconChainError::InvalidSlot(_) => {
                                 warp_utils::reject::custom_bad_request(format!(
                                     "Unable to produce payload attestation data: {e:?}"
                                 ))