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/RaftMembershipParityTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.Tests;
+
+public class RaftMembershipParityTests
+{
+    [Fact]
+    public void Membership_changes_update_node_membership_state()
+    {
+        var node = new RaftNode("N1");
+        node.AddMember("N2");
+        node.AddMember("N3");
+        node.Members.ShouldContain("N2");
+        node.Members.ShouldContain("N3");
+
+        node.RemoveMember("N2");
+        node.Members.ShouldNotContain("N2");
+    }
+}