Fix handling of VC validators by per payload-builder payload builder …

…validator registration (issue 5599).

The proposed change ensures that VC validators are registered with the builder specified by the `--payload-builder-url` argument even if the beacon node has no attached validators. It also prevent such validators from being unintentionally registered with builders configured for specific attached validators by the keymanager api.

beacon_chain/validators/beacon_validators.nim

@@ -1576,7 +1576,8 @@ proc registerValidatorsPerBuilder(
 
     # First, check for VC-added keys; cheaper because provided pre-signed
     var nonExitedVcPubkeys: HashSet[ValidatorPubKey]
-    if node.externalBuilderRegistrations.len > 0:
+    if  node.externalBuilderRegistrations.len > 0 and
+        payloadBuilderAddress == node.config.getPayloadBuilderAddress.value:
       withState(node.dag.headState):
         let currentEpoch = node.currentSlot().epoch
         for i in 0 ..< forkyState.data.validators.len:
@@ -1663,6 +1664,12 @@ proc registerValidators*(node: BeaconNode, epoch: Epoch) {.async.} =
   if not node.config.payloadBuilderEnable: return
 
   var builderKeys: Table[string, seq[ValidatorPubKey]]
+
+  # Ensure VC validators are still registered if we have no attached validators
+  let externalPayloadBuilderAddress = node.config.getPayloadBuilderAddress
+  if externalPayloadBuilderAddress.isSome:
+    builderKeys[externalPayloadBuilderAddress.value] = newSeq[ValidatorPubKey](0)
+
   for pubkey in node.attachedValidators[].validators.keys:
     let payloadBuilderAddress = node.getPayloadBuilderAddress(pubkey).valueOr:
       continue