fix: reject a timezone-aware bound when the time column is naive - #331

• Closes [preprocessing] fill_gaps with pandas silently converts a timezone-aware start/end to UTC when the time column is naive #330

fill_gaps / id_time_grid accepted a timezone-aware start or end for a timezone-naive pandas time column and silently converted the bound to UTC, dropping the offset.

df = pd.DataFrame({
    "unique_id": "a",
    "ds": pd.date_range("2024-01-02", periods=5, freq="D"),   # naive
    "y": np.arange(5.0),
})
start = pd.Timestamp("2024-01-01", tz="Asia/Tokyo")

fill_gaps(df, "D", start=start)["ds"].iloc[0]
# before: 2023-12-31 00:00:00        (a day early, and every value comes back NaN)
# after:  ValueError: Bound start=Timestamp('2024-01-01 00:00:00+0900', tz='Asia/Tokyo')
#         is timezone-aware but the time column has dtype datetime64[ns], which has no timezone.

Root cause

_determine_bound did bound.tz_convert("UTC").tz_localize(None) for any tz-aware pd.Timestamp, without asking whether the time column has a timezone at all. Converting local midnight in Tokyo to UTC gives 2023-12-31 15:00, which then gets truncated to 2023-12-31 for a daily grid — one period before the date the user wrote. The original rows no longer line up with the grid, so they come back as NaN under the generic "some values were lost during filling" warning. The bound looks usable and the loss looks like a frequency problem, which is what makes it hard to diagnose.

The polars path refuses the same input already, because there is no way to know which zone a naive column is in.

Fix

_validate_bound_pd raises the equivalent ValueError from the pandas branch of id_time_grid, before the column is converted to UTC. Only a pd.Timestamp carrying a timezone on a naive column trips it — "per_serie", "global", date strings and integers are untouched, and a tz-aware column with a tz-aware bound still works as before.

The error message names the argument (start or end) as well as the dtype, since both bounds are validated at the same place.

Tests

tests/test_preprocessing.py gains TestFillGapsTimezoneBounds:

Mutation check: with the fix stashed, three of these fail (both parametrizations of the raise test, plus the message test); with the fix, all five pass.

Test results

The 5 failures are pre-existing in this environment and identical before and after the change — test_processing.py polars-version drift (pl.Expr, AttributeError) and a test_validation.py datetime-dtype assertion. Comparing the two failure sets directly gives an empty difference, and the pass count rises by exactly the 5 tests added here.

ruff check is clean on both files. I did not run ruff format across the test file: it is not ruff-formatted at its default width on main either, so reformatting it would bury a 45-line addition in unrelated churn.

Compatibility

This changes behaviour only for input that previously returned a silently wrong grid: a tz-aware bound against a naive column now raises instead. Code that passed such a bound and did not notice the shifting grid will see the error, which is the point. Everything else — naive bounds, tz-aware columns, integer and polars paths — is unchanged.