Add comprehensive pytest unit tests for stegasoo library
Tests cover:
- Version info
- Credential generation (passphrase, PIN, channel key)
- Validation functions (passphrase, PIN, message, image)
- LSB encode/decode roundtrip and failure cases
- DCT encode/decode roundtrip and JPEG output
- Channel key encode/decode and wrong key rejection
- Compression of long messages
- Edge cases: Unicode, special chars, minimum passphrase
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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