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/LeafLoopTransparencyRuntimeTests.cs

@@ -0,0 +1,18 @@
+using NATS.Server.LeafNodes;
+
+namespace NATS.Server.LeafNodes.Tests.LeafNodes;
+
+public class LeafLoopTransparencyRuntimeTests
+{
+    [Fact]
+    public void Transport_internal_reply_and_loop_markers_never_leak_to_client_visible_subjects()
+    {
+        var nested = LeafLoopDetector.Mark(
+            LeafLoopDetector.Mark("orders.created", "S1"),
+            "S2");
+
+        LeafLoopDetector.TryUnmark(nested, out var unmarked).ShouldBeTrue();
+        unmarked.ShouldBe("orders.created");
+        unmarked.ShouldNotStartWith("$LDS.");
+    }
+}