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/RaftSnapshotCatchupTests.cs

@@ -0,0 +1,16 @@
+namespace NATS.Server.Raft.Tests;
+
+public class RaftSnapshotCatchupTests
+{
+    [Fact]
+    public async Task Lagging_follower_catches_up_via_snapshot()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        await cluster.GenerateCommittedEntriesAsync(500);
+
+        await cluster.RestartLaggingFollowerAsync();
+        await cluster.WaitForFollowerCatchupAsync();
+
+        cluster.LaggingFollower.AppliedIndex.ShouldBe(cluster.Leader.AppliedIndex);
+    }
+}