[Bug]: keep.add_time_to_date silently returns the input date when it cannot read the time string

add_time_to_date pulls units out with re.findall(r"(\d+)([wdhms])", time_str) and adds whatever it found. When it finds nothing it adds nothing and returns the date it was given, so a string it does not understand behaves exactly like "add zero".

from keep.functions import add_time_to_date
d, f = "2024-07-01", "%Y-%m-%d"

add_time_to_date(d, f, "2d")        # 2024-07-03
add_time_to_date(d, f, "1y")        # 2024-07-01   unchanged
add_time_to_date(d, f, "3M")        # 2024-07-01   unchanged
add_time_to_date(d, f, "10")        # 2024-07-01   unchanged
add_time_to_date(d, f, "2 d")       # 2024-07-01   unchanged
add_time_to_date(d, f, "")          # 2024-07-01   unchanged

Years and months are the ones I would expect people to reach for, and neither is a supported unit. A workflow computing an SLA deadline or a snooze window from 1y gets the start date back and nothing anywhere says the input was not understood, so the workflow looks like it worked.

m already means minutes, so 3M cannot be added as months without making the unit ambiguous. This is only about the silence, not about supporting more units.

Expected: a time string with no readable unit raises, the way an unreadable value does elsewhere in this module. Everything the regex accepts today keeps working, spaces included: 1w, 2d, 3h, 30m, 45s, 1w 2d 3h 30m.

I have a fix and tests.