When SeekableSharpCompressStream.StopRecording() was called (e.g. after the tar
probe in GZipFactory.TryOpenReader), it cleared _recordedPosition without seeking
back to it. This left the underlying FileStream at the advanced position reached
during the probe, so the subsequent GZipReader would start reading from the
wrong offset, producing Key=null and 0 extracted bytes.
Fix: seek back to _recordedPosition before clearing it, matching the behavior of
the non-seekable SharpCompressStream.StopRecording() which rewinds _logicalPosition.
Also enhance the existing GZip_ReaderFactory_FlatGZip test to actually verify the
extracted content (not just MoveToNextEntry returns true), and add an async variant.
Closes#1391
