feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/RaftConsensusAdvancedParityTests.cs

@@ -0,0 +1,22 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests;
+
+public class RaftConsensusAdvancedParityTests
+{
+    [Fact]
+    public async Task Leader_heartbeats_keep_followers_current_and_next_index_backtracks_on_mismatch()
+    {
+        var transport = new InMemoryRaftTransport();
+        var leader = new RaftNode("L", transport);
+        var follower = new RaftNode("F", transport);
+        transport.Register(leader);
+        transport.Register(follower);
+
+        await transport.AppendHeartbeatAsync("L", ["F"], term: 2, default);
+        follower.Term.ShouldBe(2);
+
+        RaftReplicator.BacktrackNextIndex(5).ShouldBe(4);
+        RaftReplicator.BacktrackNextIndex(1).ShouldBe(1);
+    }
+}