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/RaftSafetyContractTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Raft.Tests;
+
+public class RaftSafetyContractTests
+{
+    [Fact]
+    public async Task Follower_rejects_stale_term_vote_and_append()
+    {
+        var node = new RaftNode("n1");
+        node.StartElection(clusterSize: 1);
+
+        var staleVote = node.GrantVote(term: node.Term - 1);
+        staleVote.Granted.ShouldBeFalse();
+
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await node.TryAppendFromLeaderAsync(new RaftLogEntry(1, node.Term - 1, "cmd"), default));
+    }
+}