Fix async test failures after xunit v3 upgrade

[Copilot]

The xunit v3 upgrade exposed that async tests were not properly awaiting async operations. Tests were passing ValueTask<IAsyncReader> directly to await using statements instead of awaiting the task first, and ReaderFactory.OpenAsyncReader was calling synchronous IsArchive methods during format detection.

Changes

• Made ReaderFactory.OpenAsyncReader properly async: Changed return type to ValueTask<IAsyncReader> and replaced synchronous IsArchive() calls with IsArchiveAsync() during format detection
• Fixed test await patterns: Added explicit await before ReaderFactory.OpenAsyncReader() in all async test methods
• Kept AsyncOnlyStream in Archive async tests: Archive async tests continue to use AsyncOnlyStream to validate that archive operations properly handle async-only streams, exposing areas where synchronous I/O is still used in async code paths

Example

Before:

await using var reader = ReaderFactory.OpenAsyncReader(stream); // Sync IsArchive() called internally

After:

await using var reader = await ReaderFactory.OpenAsyncReader(stream); // Async IsArchiveAsync() called

Known Limitations

• Archive APIs: Archive opening performs synchronous I/O during format probing, causing tests with AsyncOnlyStream to fail. These tests intentionally expose the need for async archive opening support.
• Compressed tar formats: Reader opening for compressed tar formats (tar.bz2, tar.lz, tar.gz) requires multi-step format probing not yet implemented in the async path, causing "Cannot determine compressed stream type" errors.

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