Fix SevenZipArchive.IsSolidAsync() always returning false

[Copilot]

IAsyncArchive.IsSolidAsync() always returned false for 7z archives regardless of archive structure, while IArchive.IsSolid returned the correct value.

Root cause

AbstractArchive.Async.cs provides a base implementation that always returns false:

public virtual ValueTask<bool> IsSolidAsync() => new(false);

SevenZipArchive overrides the sync IsSolid property correctly (checking if any compressed folder contains more than one file), but was missing the async counterpart. RarArchive already has its own IsSolidAsync() override.

Changes

• SevenZipArchive.Async.cs: Added IsSolidAsync() override using async enumeration with the same logic as the sync property. Uses Skip(1).Any() rather than Count() > 1 to short-circuit on the second element:

public override async ValueTask<bool> IsSolidAsync()
{
    var entries = await EntriesAsync
        .Where(x => !x.IsDirectory)
        .ToListAsync()
        .ConfigureAwait(false);
    return entries.GroupBy(x => x.FilePart.Folder).Any(folder => folder.Skip(1).Any());
}

• SevenZipArchive.cs: Applied the same Skip(1).Any() optimization to the sync IsSolid property.

• SevenZipArchiveAsyncTests.cs: Added SevenZipArchive_TestSolidDetectionAsync to verify async detection matches sync detection across solid, single-block solid, and non-solid archives. Updated existing async tests to use IsSolidAsync() directly instead of casting to the concrete type to access the sync property.