feat: add randomized election timeout jitter (Gap 8.8)

Add RandomizedElectionTimeout() method to RaftNode returning TimeSpan in
[ElectionTimeoutMinMs, ElectionTimeoutMaxMs) using TotalMilliseconds (not
.Milliseconds component) to prevent synchronized elections after partitions.
Make Random injectable for deterministic testing. Fix SendHeartbeatAsync stub
in NatsRaftTransport and test-local transport implementations to satisfy the
IRaftTransport interface added in Gap 8.7.

tests/NATS.Server.Tests/Raft/RaftStrictConsensusRuntimeTests.cs

@@ -43,5 +43,8 @@ public class RaftStrictConsensusRuntimeTests
 
         public Task SendTimeoutNowAsync(string leaderId, string targetId, ulong term, CancellationToken ct)
             => Task.CompletedTask;
+
+        public Task SendHeartbeatAsync(string leaderId, IReadOnlyList<string> followerIds, int term, Action<string> onAck, CancellationToken ct)
+            => Task.CompletedTask;
     }
 }