Report the codec, not 7zAES, as an encrypted 7z entry's CompressionType

7-Zip writes 7zAES as the first coder of an encrypted folder, in front
of the codec. SevenZipFilePart.GetCompression() looked at the first
coder only, so CompressionType threw InvalidFormatException for every
entry of a 7-Zip-made encrypted archive. Skip the AES filter and report
the first real coder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/SharpCompress/Common/SevenZip/SevenZipFilePart.cs

@@ -119,7 +119,11 @@ internal class SevenZipFilePart : FilePart
             return CompressionType.None;
         }
 
-        var coder = Folder.NotNull()._coders.First();
+        // 7zAES is a filter in front of the codec (7-Zip writes it as coder 0), so it says
+        // nothing about the compression; report the first real coder instead.
+        var coder =
+            Folder.NotNull()._coders.FirstOrDefault(c => c._methodId._id != CMethodId.K_AES_ID)
+            ?? throw new InvalidFormatException();
         return coder._methodId._id switch
         {
             K_COPY => CompressionType.None,

tests/SharpCompress.Test/SevenZip/SevenZipArchiveTests.cs

@@ -105,6 +105,29 @@ public class SevenZipArchiveTests : ArchiveTests
             new SevenZipFactory()
         );
 
+    [Theory]
+    [InlineData("7Zip.LZMA.Aes.7z")]
+    [InlineData("7Zip.LZMA2.Aes.7z")]
+    public void SevenZipArchive_Aes_ReportsCompressionType(string testArchive)
+    {
+        // 7-Zip writes 7zAES as the folder's first coder, in front of the codec.
+        using var stream = File.OpenRead(Path.Combine(TEST_ARCHIVES_PATH, testArchive));
+        using var archive = SevenZipArchive.OpenArchive(
+            stream,
+            ReaderOptions.ForExternalStream with
+            {
+                Password = "testpassword",
+            }
+        );
+        var files = archive.Entries.Where(entry => !entry.IsDirectory && entry.Size > 0).ToList();
+        Assert.NotEmpty(files);
+        foreach (var entry in files)
+        {
+            Assert.True(entry.IsEncrypted, entry.Key);
+            Assert.Equal(CompressionType.LZMA, entry.CompressionType);
+        }
+    }
+
     [Fact]
     public void SevenZipArchive_LZMA2AES_StreamRead() =>
         ArchiveStreamRead(