Fix SeekableSharpCompressStream.StopRecording() to rewind to recorded…

[adamhathcock]

… position

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

Supersedes #1392

[Copilot]

Pull request overview

This pull request fixes an incorrect stream position after format probing when using ReaderFactory on flat .gz files backed by a seekable stream. The change ensures SeekableSharpCompressStream.StopRecording() rewinds to the recorded anchor position before clearing it, preventing GZipReader from starting at an advanced offset (which previously could yield Key=null and 0 extracted bytes).

Changes:

• Fix SeekableSharpCompressStream.StopRecording() to seek back to the recording anchor before clearing it.
• Strengthen the existing flat-GZip ReaderFactory test to validate extracted payload bytes.
• Add an async counterpart test that validates the same behavior via OpenAsyncReader / async extraction.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/SharpCompress/IO/SeekableSharpCompressStream.cs	Rewinds to the recording anchor on StopRecording() to match non-seekable rewind-on-stop semantics and prevent incorrect downstream read offsets.
tests/SharpCompress.Test/GZip/GZipReaderTests.cs	Enhances the flat .gz ReaderFactory test to assert extracted content correctness (not just entry enumeration).
tests/SharpCompress.Test/GZip/GZipReaderAsyncTests.cs	Adds an async flat .gz ReaderFactory test validating async extraction produces the correct bytes.