Add unit tests and refactor date cleaning into function

[hl3986-wq]

What I changed

• Added clean_mta_df() in utils.py to convert the 'date' column to datetime and sort by date.
• Added pytest unit tests in tests/test_utils.py (2 tests).
• Refactored the notebook to use clean_mta_df().

How to run tests

• python -m pytest -q

[hl3968-hue]

Looks good! A few improvements in the latest commit:

1. Extracted the plotting logic into a plot_ridership_recovery() function in utils.py so the 2 notebook stays clean and the function is reusable
2. Added more tests — went from 2 to 6, now covering: original df not being mutated, already-sorted data, plot function returning a valid figure, and missing columns raising errors
   3.Added comments in tests to clarify what each assertion is checking (e.g. the reset_index behavior)