fix: add support for 'd' (days) unit in parse_duration

[Ishant5436]

/claim #1

Fixes the missing d (days) unit in parse_duration by adding "d": 86400 to the _UNITS dictionary.

Also added regression tests for 1d and 2d4h in tests/test_duration_utils.py to ensure it works accurately and handles combined units correctly.

All 9 tests passed locally:

python3 -m unittest discover -s tests
.........
----------------------------------------------------------------------
Ran 9 tests in 0.000s

OK