add more tests

tests/SharpCompress.Test/SevenZip/SevenZipWriterNonSeekableTests.cs

@@ -0,0 +1,25 @@
+using System.IO;
+using SharpCompress.Common;
+using SharpCompress.Test.Mocks;
+using SharpCompress.Writers.SevenZip;
+using Xunit;
+
+namespace SharpCompress.Test.SevenZip;
+
+/// <summary>
+/// 7z writing requires a seekable output stream so the signature header can be back-patched on
+/// finalize (see SevenZipWriter.cs). Unlike Zip, it cannot fall back to a streaming layout, so
+/// constructing a writer over a non-seekable output must fail fast. This pins that documented
+/// limitation against silent regression.
+/// </summary>
+public class SevenZipWriterNonSeekableTests
+{
+    [Fact]
+    public void SevenZip_NonSeekable_Output_Throws()
+    {
+        using var ms = new MemoryStream();
+        Assert.Throws<ArchiveOperationException>(() =>
+            new SevenZipWriter(new ForwardOnlyStream(ms), new SevenZipWriterOptions())
+        );
+    }
+}