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/RaftLeadershipTransferTests.cs

@@ -332,6 +332,9 @@ file sealed class BlockingTimeoutNowTransport : IRaftTransport
         await using var reg = ct.Register(() => tcs.TrySetCanceled(ct));
         await tcs.Task;
     }
+
+    public Task SendHeartbeatAsync(string leaderId, IReadOnlyList<string> followerIds, int term, Action<string> onAck, CancellationToken ct)
+        => Task.CompletedTask;
 }
 
 /// <summary>
@@ -408,4 +411,7 @@ file sealed class VoteGrantingTransport : IRaftTransport
 
         return Task.CompletedTask;
     }
+
+    public Task SendHeartbeatAsync(string leaderId, IReadOnlyList<string> followerIds, int term, Action<string> onAck, CancellationToken ct)
+        => Task.CompletedTask;
 }