🧪 testing improvement description

[google-labs-jules]

🎯 What: The testing gap for src/nmea/zda.py was addressed by adding tests/test_nmea/test_zda.py. The module contains decoding functions for ZDA and surprisingly GGA NMEA messages.
📊 Coverage: The new tests cover zdaEpochSeconds, zdaDecode, ggaDecode, and zdaDict2TIMESTAMP. Tested happy paths for valid messages, edge cases like negative coordinates (S/E), validation logic, and error handling for invalid lengths/starts.
✨ Result: Test coverage for src/nmea/zda.py went from 40% to 100%. No regressions were introduced in the test suite.

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PR created automatically by Jules for task 14629923030356858883 started by @schwehr

[google-labs-jules]

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