Fix logic and robustness of DataFlow._subtract_month

[mysticBliss]

This PR addresses critical issues in DataFlow._subtract_month:

1. Logic Fix: Corrected the year calculation formula when subtracting months that result in a previous year. The original code used year - y, which erroneously increased the year when y was negative. It is now year + y.
2. Robustness: Added handling for month-end dates. Previously, subtracting a month from a date like March 31st would crash with ValueError if the previous month had fewer days. The new implementation clamps the day to the last valid day of the target month using calendar.monthrange.
3. Testing: Expanded tests/test_data_flow.py with unit tests covering normal subtraction, year wrap-around, and month-end edge cases.

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PR created automatically by Jules for task 2013603253886078263 started by @mysticBliss

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