🧪 Add missing UnicodeDecodeError test in _decode_event

[manupawickramasinghe]

🎯 What: The testing gap addressed
The error handling block for UnicodeDecodeError in _decode_event in python/wifi_densepose/client/primitives.py was missing test coverage.

📊 Coverage: What scenarios are now tested
Added test_listener_decodes_invalid_utf8_bytes_payload to python/tests/test_client_primitives.py which passes an invalid utf-8 byte sequence (b"\xff\xfe\xff") as payload.

✨ Result: The improvement in test coverage
The fallback case of UnicodeDecodeError when decoding the MQTT event payload correctly triggers and is now tested to ensure it returns an empty state string and empty raw dictionary.

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PR created automatically by Jules for task 13141292057724951611 started by @manupawickramasinghe

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