test: add comprehensive unit tests

[srleach]

Summary

Add comprehensive unit tests for the spa2mqtt project.

Test Files

test_packet.py

• Basic packet parsing and validation
• Checksum verification
• Protocol integration

test_protocols.py

• Protocol abstraction (Plaintext, JacuzziEncrypted)
• Protocol registry and lookup
• Body decoding

test_base.py

• Message field decoding (scale, xor, shift, mask, output_map)
• MessageFactory registration
• Spa base class initialization
• Communicator setup

test_jacuzzi_encrypted.py

• JacuzziEncryptedPacket construction and enum parsing
• JacuzziEncryptedPacketType values
• JacuzziTopsideControllerButton enum
• JacuzziEncryptedSpa initialization
• JacuzziEncryptedCommunicator
• Config loading from YAML

test_transport.py

• Transport layer (load_class, make_communicator, get_communicator)
• Spa configuration loading
• Variant configuration resolution

Coverage

• 57 tests passing
• 5 skipped (message decoding tests require valid packet checksums to set up)
• Tests cover pluggable communicator types (base, jacuzzi_encrypted)

Running Tests

pytest tests/ -v