fix(functions): refuse a time string add_time_to_date cannot read

[chiruu12]

Fixes #6778

add_time_to_date reads units with re.findall(r"(\d+)([wdhms])", time_str) and adds what it finds. When it finds nothing it adds nothing, so a string it cannot read returns the date unchanged and looks like it worked.

This raises instead. The message names the string it could not read and the units it understands.

Nothing that parsed before changes. 1w, 2d, 3h, 30m, 45s and 1w 2d 3h 30m all still work; the guard fires only when the regex matched nothing at all, which is why the spaced form is unaffected.

Tests cover six unreadable spellings, including 1y and 3M, which are the units people reach for and neither of which is supported. Three more pin the accepted forms as controls. The six fail on the commit before the fix and the three pass either way.

tests/test_functions.py tests/test_iohandler.py: 206 passed. ruff check clean.

I did not add month support. m already means minutes, so 3M cannot be a month without making the unit ambiguous. This is about the silence, not the unit set.

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