refactor: extract NATS.Server.Raft.Tests project

Move 43 Raft consensus test files (8 root-level + 35 in Raft/ subfolder)
from NATS.Server.Tests into a dedicated NATS.Server.Raft.Tests project.
Update namespaces, add InternalsVisibleTo, and fix timing/exception
handling issues in moved test files.

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

@@ -0,0 +1,16 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.Tests;
+
+public class RaftConsensusRuntimeParityTests
+{
+    [Fact]
+    public async Task Raft_cluster_commits_with_next_index_backtracking_semantics()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        await cluster.GenerateCommittedEntriesAsync(5);
+        await cluster.WaitForAppliedAsync(5);
+
+        cluster.Nodes.All(n => n.AppliedIndex >= 5).ShouldBeTrue();
+    }
+}