Force validation in block downloader (#3413)

* forceValidation

* Fix Ethash edge case (<= rather than <), In spec its >

* randomNumberValidation

* Max 0

* Rename target

* Added test

* cosmetic in the test

Co-authored-by: lukasz.rozmej <lukasz.rozmej@gmail.com>

src/Nethermind/Nethermind.Consensus.Ethash/Ethash.cs

@@ -151,11 +151,11 @@ private static ulong GetRandomNonce()
             return BitConverter.ToUInt64(buffer, 0);
         }
 
-        private bool IsLessThanTarget(byte[] result, UInt256 difficulty)
+        private bool IsLessOrEqualThanTarget(byte[] result, UInt256 difficulty)
         {
             UInt256 resultAsInteger = new(result, true);
-            BigInteger threshold = BigInteger.Divide(_2To256, (BigInteger)difficulty);
-            return (BigInteger)resultAsInteger < threshold;
+            BigInteger target = BigInteger.Divide(_2To256, (BigInteger)difficulty);
+            return (BigInteger)resultAsInteger <= target;
         }
 
         public (Keccak MixHash, ulong Nonce) Mine(BlockHeader header, ulong? startNonce = null)
@@ -178,7 +178,7 @@ private bool IsLessThanTarget(byte[] result, UInt256 difficulty)
             {
                 byte[] result;
                 (mixHash, result, _) = Hashimoto(fullSize, dataSet, headerHashed, null, nonce);
-                if (IsLessThanTarget(result, header.Difficulty))
+                if (IsLessOrEqualThanTarget(result, header.Difficulty))
                 {
                     break;
                 }
@@ -243,7 +243,7 @@ public bool Validate(BlockHeader header)
                 return false;
             }
 
-            return IsLessThanTarget(result, header.Difficulty);
+            return IsLessOrEqualThanTarget(result, header.Difficulty);
         }
 
         private readonly Stopwatch _cacheStopwatch = new();