Fix multi-channel SSH operations by respecting session I/O needs

[krystophny]

Summary

Fixes assertion failures when using multiple channels on a single SSH session, enabling proper async I/O multiplexing.

Problem

Applications like SSHBind that create multiple channels on a single SSH session were hitting assertion failures in the async I/O code:

assertion failed: expected_block_directions.is_readable()

The issue occurred because the code was asserting that sess.block_directions() (session's aggregate I/O needs) matched expected_block_directions (individual operation expectations). With multiple channels, these naturally differ.

Solution

Always wait for whatever I/O the session needs, regardless of what the current operation expects:

• Remove assertions checking expected_block_directions matches sess.block_directions()
• Always respect sess.block_directions() which represents the aggregate state of all channels
• Enable proper async I/O multiplexing where operations cooperate to make progress

Changes

• Updated impl_tokio.rs and impl_async_io.rs to remove blocking assertions
• Added comprehensive test suite (test_audit_multichannel.rs) covering:
  • Sequential channel operations
  • Concurrent interleaved operations
  • Stress testing with multiple channels
  • Data integrity verification
  • Error handling scenarios

Testing

All existing tests pass. New tests verify:

• ✅ No data corruption between channels
• ✅ Proper async serialization of operations
• ✅ SSHBind-style usage patterns work correctly
• ✅ Error conditions handled gracefully

Compatibility

• Single-threaded async: Fully supported
• Sequential operations: Full compatibility maintained
• Multi-threaded: Still requires external synchronization due to libssh2 limitations

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