refactor: extract NATS.Server.LeafNodes.Tests project

Move 28 leaf node test files from NATS.Server.Tests into a dedicated
NATS.Server.LeafNodes.Tests project. Update namespaces, add
InternalsVisibleTo, register in solution file. Replace all Task.Delay
polling loops with PollHelper.WaitUntilAsync/YieldForAsync from
TestUtilities. Replace private ReadUntilAsync in LeafProtocolTests
with SocketTestHelper.ReadUntilAsync.

All 281 tests pass.

tests/NATS.Server.LeafNodes.Tests/LeafNodes/LeafHubSpokeMappingParityTests.cs

@@ -0,0 +1,21 @@
+using NATS.Server.LeafNodes;
+
+namespace NATS.Server.LeafNodes.Tests.LeafNodes;
+
+public class LeafHubSpokeMappingParityTests
+{
+    [Fact]
+    public void Leaf_hub_spoke_mapper_round_trips_account_mapping()
+    {
+        var mapper = new LeafHubSpokeMapper(new Dictionary<string, string>
+        {
+            ["HUB"] = "SPOKE",
+        });
+
+        var outbound = mapper.Map("HUB", "orders.created", LeafMapDirection.Outbound);
+        outbound.Account.ShouldBe("SPOKE");
+
+        var inbound = mapper.Map("SPOKE", "orders.created", LeafMapDirection.Inbound);
+        inbound.Account.ShouldBe("HUB");
+    }
+}