Fix GZip extraction NotSupportedException for non-seekable streams

[Copilot]

GZip decompression fails when reading from non-seekable streams (e.g., HttpClient.GetStreamAsync()) because GZipFilePart unconditionally accesses stream.Position, which throws NotSupportedException on HTTP streams.

Changes

• GZipFilePart.cs: Check stream.CanSeek before accessing Position. Set EntryStartPosition = 0 for non-seekable streams since they read sequentially from current position.

• GZipArchiveEntry.cs: Check CanSeek before attempting position reset in OpenEntryStream().

• Test coverage: Added GZip_Archive_NonSeekableStream test using a NonSeekableStream wrapper to verify the fix.

Usage

// Now works with HTTP streams
using var httpStream = await httpClient.GetStreamAsync(gzipUrl);
using var reader = GZipReader.Open(httpStream);
while (reader.MoveToNextEntry())
{
    reader.WriteEntryTo(outputStream);
}

Note: GZipReader supports non-seekable streams. GZipArchive.Open() continues to require seekable streams by design.

Original prompt

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This section details on the original issue you should resolve

<issue_title>HttpBaseStream Gzip extract - NotSupportedException</issue_title>
<issue_description>Hi, looks like when trying to decompress a gzip through httpStream it throws NotSupportedException on Position when setting EntryStartPosition. what is the purpose of settings this field? how can i mitigate the problem? Thanks!

System.NotSupportedException: Specified method is not supported.
   at System.Net.Http.HttpBaseStream.get_Position()
   at SharpCompress.IO.RewindableStream.get_Position()
   at SharpCompress.Common.GZip.GZipFilePart..ctor(Stream stream, ArchiveEncoding archiveEncoding)
   at SharpCompress.Common.GZip.GZipEntry.GetEntries(Stream stream, OptionsBase options)+MoveNext()

P.S i'd prefer to not save the file to disk and read it again as FileStream if possible
If that's not possible which types require random access? i know that .tar.gz works perfectly fine through http for example, Thanks in advance!</issue_description>

Comments on the Issue (you are @copilot in this section)

@adamhathcock You'll have to show me how you're calling things.

GZipFilePart does check CanSeek but Rewindable is meant to buffer unbufferable streams so shouldn't be used here.</comment_new>
<comment_new>@adamhathcock
Actually, I take that back, GZipFilePart has a usage of Position no matter seeking. That should be changed</comment_new>
<comment_new>@adamhathcock
you're right but I'm not sure why I'm doing that....I'd have to look and remember</comment_new>

• Fixes HttpBaseStream Gzip extract - NotSupportedException #843

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