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/RaftSnapshotTransferParityTests.cs

@@ -0,0 +1,25 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.Tests;
+
+public class RaftSnapshotTransferParityTests
+{
+    [Fact]
+    public async Task Snapshot_transfer_installs_snapshot_when_follower_falls_behind()
+    {
+        var transport = new InMemoryRaftTransport();
+        var leader = new RaftNode("L", transport);
+        var follower = new RaftNode("F", transport);
+        transport.Register(leader);
+        transport.Register(follower);
+
+        var snapshot = new RaftSnapshot
+        {
+            LastIncludedIndex = 10,
+            LastIncludedTerm = 3,
+        };
+
+        await transport.InstallSnapshotAsync("L", "F", snapshot, default);
+        follower.AppliedIndex.ShouldBe(10);
+    }
+}