Fix ReadFullyAsync with ArrayPool buffer in SevenZipArchive signature check

[Copilot]

ArrayPool.Rent(6) may return buffers larger than 6 bytes, but ReadFullyAsync(buffer, cancellationToken) attempts to fill the entire buffer, causing reads beyond the intended 6-byte signature.

Changes

• Changed ReadFullyAsync(buffer, cancellationToken) to ReadFullyAsync(buffer, 0, 6, cancellationToken) in SignatureMatchAsync
• Now consistent with synchronous SignatureMatch which correctly uses ReadExact(buffer, 0, 6)

// Before: reads buffer.Length bytes (could be > 6)
if (!await stream.ReadFullyAsync(buffer, cancellationToken).ConfigureAwait(false))

// After: reads exactly 6 bytes
if (!await stream.ReadFullyAsync(buffer, 0, 6, cancellationToken).ConfigureAwait(false))

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