Fix flat GZip via ReaderFactory: SeekableSharpCompressStream.StopRecording() must seek back to anchor

[Copilot]

ReaderFactory.OpenReader on a flat .gz file (non-tar) produced Key=null and extracted 0 bytes because SeekableSharpCompressStream.StopRecording() cleared _recordedPosition without seeking back to it.

After GZipFactory.TryOpenReader runs the tar probe (decompressing the entire GZip stream to check for a tar header), the underlying FileStream is left at an advanced position. StopRecording() must rewind before clearing the anchor — the non-seekable SharpCompressStream already does this by resetting _logicalPosition, but the seekable override was missing the seek.

Changes

• SeekableSharpCompressStream.StopRecording() — seek to _recordedPosition before clearing it, matching the non-seekable implementation's behavior of rewinding to the recording anchor on stop.

// Before (bug): position stays wherever the tar probe left it
public override void StopRecording() => _recordedPosition = null;

// After (fix): rewind first, matching non-seekable SharpCompressStream semantics
public override void StopRecording()
{
    if (_recordedPosition.HasValue)
        _stream.Seek(_recordedPosition.Value, SeekOrigin.Begin);
    _recordedPosition = null;
}

• GZipReaderTests.GZip_ReaderFactory_FlatGZip — extended to verify the extracted bytes, not just that MoveToNextEntry() returns true (the prior assertion missed the actual regression).
• GZipReaderAsyncTests.GZip_ReaderFactory_FlatGZip_Async — async equivalent of the above.

• Fixes Flat GZip via ReaderFactory broken: stream seems not to be rewound after tar probe in GZipFactory.TryOpenReader #1391