Add comprehensive RAR5 async extraction tests

[Copilot]

The async RAR5 extraction code path (Unpack5Async) lacked dedicated test coverage to verify it executes correctly vs. falling back to sync methods.

Changes

• New test suite: Rar5AsyncExtractionTests with 12 tests covering:

  • Basic and solid archive extraction
  • Encrypted archives (header+files, files-only)
  • Special formats (Blake2 CRC, comments, uncompressed)
  • Multiple extraction methods (ExtractAllEntries, WriteToDirectory, OpenEntryStream)
  • Entry skipping in solid archives

• Async enforcement: All tests use AsyncOnlyStream wrapper that throws on synchronous Read operations, ensuring async code paths execute rather than sync fallbacks

// AsyncOnlyStream forces async execution
using var stream = File.OpenRead("Rar5.solid.rar");
await using var reader = await ReaderFactory.OpenAsyncReader(new AsyncOnlyStream(stream));

while (await reader.MoveToNextEntryAsync())
{
    if (!reader.Entry.IsDirectory)
    {
        await reader.WriteEntryToDirectoryAsync(outputPath);
    }
}

All tests pass on net48 and net10.0.

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