Release owned Squeeze buffers and failed PPMd models

tests/SharpCompress.Test/Streams/SqueezeLifetimeTests.cs

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+using System;
+using System.IO;
+using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
+using SharpCompress.Compressors.PPMd;
+using SharpCompress.Compressors.Squeezed;
+using Xunit;
+
+namespace SharpCompress.Test.Streams;
+
+public class SqueezeLifetimeTests
+{
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task DisposeReleasesOwnedHuffmanStream(bool asyncOpen)
+    {
+        // One Huffman node: literal A on zero, end marker on one.
+        using var source = new MemoryStream(new byte[] { 1, 0, 190, 255, 255, 254, 2 });
+        var stream = asyncOpen
+            ? await SqueezeStream.CreateAsync(source, 7, CancellationToken.None)
+            : SqueezeStream.Create(source, 7);
+        var owned = (Stream)
+            typeof(SqueezeStream)
+                .GetField("_huffmanDecoded", BindingFlags.Instance | BindingFlags.NonPublic)!
+                .GetValue(stream)!;
+        Assert.True(owned.CanRead);
+        stream.Dispose();
+        Assert.False(owned.CanRead);
+        Assert.True(source.CanRead);
+    }
+
+    [Fact]
+    public void UnsupportedPpmdCompressionLeavesInputOpen()
+    {
+        using var source = new MemoryStream();
+        var properties = new PpmdProperties(new byte[] { 2, 0, 0, 16, 0 });
+        Assert.Throws<NotImplementedException>(() => PpmdStream.Create(properties, source, true));
+        Assert.True(source.CanRead);
+    }
+}