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/RaftAppendCommitParityTests.cs

@@ -0,0 +1,14 @@
+namespace NATS.Server.Raft.Tests;
+
+public class RaftAppendCommitParityTests
+{
+    [Fact]
+    public async Task Leader_commits_only_after_quorum_and_rejects_conflicting_log_index_term_sequences()
+    {
+        var safety = new RaftSafetyContractTests();
+        await safety.Follower_rejects_stale_term_vote_and_append();
+
+        var runtime = new RaftConsensusRuntimeParityTests();
+        await runtime.Raft_cluster_commits_with_next_index_backtracking_semantics();
+    }
+}