Move shared fixtures and parity utilities to TestUtilities project

- git mv JetStreamApiFixture, JetStreamClusterFixture, LeafFixture,
  Parity utilities, and TestData from NATS.Server.Tests to
  NATS.Server.TestUtilities
- Update namespaces to NATS.Server.TestUtilities (and .Parity sub-ns)
- Make fixture classes public for cross-project access
- Add PollHelper to replace Task.Delay polling with SemaphoreSlim waits
- Refactor all fixture polling loops to use PollHelper
- Add 'using NATS.Server.TestUtilities;' to ~75 consuming test files
- Rename local fixture duplicates (MetaGroupTestFixture,
  LeafProtocolTestFixture) to avoid shadowing shared fixtures
- Remove TestData entry from NATS.Server.Tests.csproj (moved to
  TestUtilities)

tests/NATS.Server.TestUtilities/Parity/NatsCapabilityInventory.cs

@@ -0,0 +1,52 @@
+namespace NATS.Server.TestUtilities.Parity;
+
+public sealed record CapabilityRow(string Capability, string Behavior, string Tests, string Docs);
+
+public sealed class NatsCapabilityInventoryReport
+{
+    public NatsCapabilityInventoryReport(IReadOnlyList<CapabilityRow> rows)
+    {
+        Rows = rows;
+    }
+
+    public IReadOnlyList<CapabilityRow> Rows { get; }
+
+    public IReadOnlyList<CapabilityRow> InvalidRows => Rows
+        .Where(r => !IsDone(r.Behavior) && IsClosed(r.Docs))
+        .Concat(Rows.Where(r => !IsDone(r.Tests) && IsClosed(r.Docs)))
+        .Distinct()
+        .ToArray();
+
+    private static bool IsDone(string status) => string.Equals(status, "done", StringComparison.OrdinalIgnoreCase);
+    private static bool IsClosed(string status) => string.Equals(status, "closed", StringComparison.OrdinalIgnoreCase);
+}
+
+public static class NatsCapabilityInventory
+{
+    public static NatsCapabilityInventoryReport Load(string relativePath)
+    {
+        var repositoryRoot = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, "..", "..", "..", "..", ".."));
+        var mapPath = Path.Combine(repositoryRoot, relativePath);
+        File.Exists(mapPath).ShouldBeTrue();
+
+        var rows = new List<CapabilityRow>();
+        foreach (var rawLine in File.ReadLines(mapPath))
+        {
+            var line = rawLine.Trim();
+            if (!line.StartsWith("|", StringComparison.Ordinal) || line.Contains("---", StringComparison.Ordinal))
+                continue;
+
+            var cells = line.Trim('|').Split('|').Select(static c => c.Trim()).ToArray();
+            if (cells.Length < 4 || string.Equals(cells[0], "Capability", StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            rows.Add(new CapabilityRow(
+                cells[0],
+                cells[1],
+                cells[2],
+                cells[3]));
+        }
+
+        return new NatsCapabilityInventoryReport(rows);
+    }
+}