Release 7.0.16-beta: fix HTTP/2 watermark test race

tests/Titanium.Web.Proxy.UnitTests/Http2FrameWriterWatermarkTests.cs

@@ -17,16 +17,21 @@ public async Task EnqueueAgainstStalledDrain_RaisesHighWaterPastSoftThreshold()
         await using var stall = new StallStream();
         await using var writer = new Http2FrameWriter(stall);
 
-        // First write blocks the drain on StallStream; subsequent frames pile up in the channel.
+        // Block the drain inside WriteAsync first. Enqueueing the backlog before that races the
+        // drain: it can coalesce a partial batch, stall, and leave HighWater under 64.
+        var primer = ArrayPool<byte>.Shared.Rent(32);
+        writer.EnqueueRented(primer, 32);
+        var writeStarted = stall.WaitForWriteStarted();
+        var started = await Task.WhenAny(writeStarted, Task.Delay(TimeSpan.FromSeconds(2)));
+        Assert.AreEqual(writeStarted, started, "drain did not reach WriteAsync");
+
         for (var i = 0; i < 80; i++)
         {
             var rented = ArrayPool<byte>.Shared.Rent(32);
             writer.EnqueueRented(rented, 32);
         }
 
-        // Drain coalesces up to 64 frames into one WriteAsync that stalls; pending drops for those
-        // frames while HighWater still recorded the backlog peak.
-        Assert.IsTrue(SpinWait.SpinUntil(() => writer.HighWaterMark >= 64, TimeSpan.FromSeconds(2)),
+        Assert.IsTrue(writer.HighWaterMark >= 64,
             $"expected high-water ≥ 64, got {writer.HighWaterMark}");
         Assert.IsTrue(writer.HighWaterMark >= writer.PendingFrameCount);
 
@@ -38,12 +43,17 @@ private sealed class StallStream : Stream
     {
         private readonly TaskCompletionSource<bool> gate =
             new(TaskCreationOptions.RunContinuationsAsynchronously);
+        private readonly TaskCompletionSource started =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
 
         public void Release() => gate.TrySetResult(true);
 
+        public Task WaitForWriteStarted() => started.Task;
+
         public override async ValueTask WriteAsync(ReadOnlyMemory<byte> buffer,
             CancellationToken cancellationToken = default)
         {
+            started.TrySetResult();
             await gate.Task.WaitAsync(cancellationToken);
         }