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/ParityRowInspector.cs

@@ -0,0 +1,67 @@
+namespace NATS.Server.TestUtilities.Parity;
+
+public sealed record ParityRow(string Section, string SubSection, string Feature, string DotNetStatus);
+
+public sealed class ParityReport
+{
+    public ParityReport(IReadOnlyList<ParityRow> rows)
+    {
+        Rows = rows;
+    }
+
+    public IReadOnlyList<ParityRow> Rows { get; }
+
+    public IReadOnlyList<ParityRow> UnresolvedRows =>
+        Rows.Where(r => r.DotNetStatus is "N" or "Baseline" or "Stub").ToArray();
+}
+
+public static class ParityRowInspector
+{
+    public static ParityReport Load(string relativePath)
+    {
+        var repositoryRoot = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, "..", "..", "..", "..", ".."));
+        var differencesPath = Path.Combine(repositoryRoot, relativePath);
+        File.Exists(differencesPath).ShouldBeTrue();
+
+        var section = string.Empty;
+        var subsection = string.Empty;
+        var rows = new List<ParityRow>();
+        foreach (var rawLine in File.ReadLines(differencesPath))
+        {
+            var line = rawLine.Trim();
+            if (line.StartsWith("## ", StringComparison.Ordinal))
+            {
+                section = line[3..].Trim();
+                continue;
+            }
+
+            if (line.StartsWith("### ", StringComparison.Ordinal))
+            {
+                subsection = line[4..].Trim();
+                continue;
+            }
+
+            if (!line.StartsWith("|", StringComparison.Ordinal))
+                continue;
+
+            if (line.Contains("---", StringComparison.Ordinal))
+                continue;
+
+            var cells = line.Trim('|').Split('|').Select(c => c.Trim()).ToArray();
+            if (cells.Length < 3)
+                continue;
+
+            // Ignore table header rows; row format is expected to contain Go and .NET status columns.
+            if (cells[0] is "Feature" or "Aspect" or "Operation" or "Signal" or "Type" or "Mechanism" or "Flag")
+                continue;
+
+            rows.Add(new ParityRow(
+                section,
+                subsection,
+                cells[0],
+                cells[2]));
+        }
+
+        return new ParityReport(rows);
+    }
+}