Fix reachable Debug.Assert in InterleavedZipPackagePartStream.Length

src/libraries/System.IO.Packaging/src/System/IO/Packaging/InterleavedZipPackagePartStream.cs

@@ -477,7 +477,6 @@ public override long Length
             get
             {
                 CheckClosed();
-                Debug.Assert(CanSeek);
 
                 long length = 0;
                 for (int pieceNumber = 0; pieceNumber < _dir.GetNumberOfPieces(); ++pieceNumber)

src/libraries/System.IO.Packaging/tests/PartPieceTests.cs

@@ -637,5 +637,21 @@ public void SetLengthRemovesPartPieces(string partName, int newLength, string re
 
             zipArchive.Dispose();
         }
+
+        [Fact]
+        public void InterleavedZipPackagePartStream_Length_ReturnsCorrectValueWhenCanSeekIsFalse()
+        {
+            using MemoryStream package = new(_partPieceSampleZipPackage);
+
+            using Package zipPackage = Package.Open(package, FileMode.Open, FileAccess.Read);
+            PackagePart partEntry = zipPackage.GetPart(new Uri("/ReadablePartPieceEntry.bin", UriKind.Relative));
+            using Stream stream = partEntry.GetStream(FileMode.Open);
+
+            // When the package is opened with FileAccess.Read, the underlying zip entry stream
+            // does not support seeking but Length should still return the correct value.
+            // ReadablePartPieceEntry.bin has 4 pieces of 16 bytes each = 64 bytes total.
+            Assert.False(stream.CanSeek);
+            Assert.Equal(64, stream.Length);
+        }
     }
 }