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/LeafFixture.cs

@@ -0,0 +1,96 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+
+namespace NATS.Server.TestUtilities;
+
+/// <summary>
+/// Shared fixture for leaf node tests that creates a hub and a spoke server
+/// connected via leaf node protocol.
+/// </summary>
+public sealed class LeafFixture : IAsyncDisposable
+{
+    private readonly CancellationTokenSource _hubCts;
+    private readonly CancellationTokenSource _spokeCts;
+
+    private LeafFixture(NatsServer hub, NatsServer spoke, CancellationTokenSource hubCts, CancellationTokenSource spokeCts)
+    {
+        Hub = hub;
+        Spoke = spoke;
+        _hubCts = hubCts;
+        _spokeCts = spokeCts;
+    }
+
+    public NatsServer Hub { get; }
+    public NatsServer Spoke { get; }
+
+    public static async Task<LeafFixture> StartAsync()
+    {
+        var hubOptions = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            LeafNode = new LeafNodeOptions
+            {
+                Host = "127.0.0.1",
+                Port = 0,
+            },
+        };
+
+        var hub = new NatsServer(hubOptions, NullLoggerFactory.Instance);
+        var hubCts = new CancellationTokenSource();
+        _ = hub.StartAsync(hubCts.Token);
+        await hub.WaitForReadyAsync();
+
+        var spokeOptions = new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            LeafNode = new LeafNodeOptions
+            {
+                Host = "127.0.0.1",
+                Port = 0,
+                Remotes = [hub.LeafListen!],
+            },
+        };
+
+        var spoke = new NatsServer(spokeOptions, NullLoggerFactory.Instance);
+        var spokeCts = new CancellationTokenSource();
+        _ = spoke.StartAsync(spokeCts.Token);
+        await spoke.WaitForReadyAsync();
+
+        await PollHelper.WaitUntilAsync(
+            () => hub.Stats.Leafs > 0 && spoke.Stats.Leafs > 0,
+            timeoutMs: 5000,
+            intervalMs: 50);
+
+        return new LeafFixture(hub, spoke, hubCts, spokeCts);
+    }
+
+    public async Task WaitForRemoteInterestOnHubAsync(string subject)
+    {
+        await PollHelper.WaitOrThrowAsync(
+            () => Hub.HasRemoteInterest(subject),
+            $"Timed out waiting for remote interest on hub for '{subject}'.",
+            timeoutMs: 5000,
+            intervalMs: 50);
+    }
+
+    public async Task WaitForRemoteInterestOnSpokeAsync(string subject)
+    {
+        await PollHelper.WaitOrThrowAsync(
+            () => Spoke.HasRemoteInterest(subject),
+            $"Timed out waiting for remote interest on spoke for '{subject}'.",
+            timeoutMs: 5000,
+            intervalMs: 50);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _spokeCts.CancelAsync();
+        await _hubCts.CancelAsync();
+        Spoke.Dispose();
+        Hub.Dispose();
+        _spokeCts.Dispose();
+        _hubCts.Dispose();
+    }
+}