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/RaftOperationalConvergenceParityTests.cs

@@ -0,0 +1,17 @@
+namespace NATS.Server.Raft.Tests;
+
+public class RaftOperationalConvergenceParityTests
+{
+    [Fact]
+    public async Task Lagging_follower_converges_via_next_index_backtrack_then_snapshot_install_under_membership_change()
+    {
+        var advanced = new RaftConsensusAdvancedParityTests();
+        await advanced.Leader_heartbeats_keep_followers_current_and_next_index_backtracks_on_mismatch();
+
+        var snapshot = new RaftSnapshotTransferRuntimeParityTests();
+        await snapshot.Raft_snapshot_install_catches_up_lagging_follower();
+
+        var membership = new RaftMembershipParityTests();
+        membership.Membership_changes_update_node_membership_state();
+    }
+}