Fix date-only range end dropping the final day

[LuShadowX]

When a search time range has a stop date with no time, the compiled DateRange end lands at midnight (00:00:00). But DateRange.end is inclusive, so a query like "Jan 1 to Jan 5" produces end = 2023-01-05 00:00:00, and anything happening on Jan 5 after midnight falls outside the range — the whole final day is effectively dropped.

dtr = DateTimeRange(
    start_date=DateTime(date=DateVal(day=1, month=1, year=2023)),
    stop_date=DateTime(date=DateVal(day=5, month=1, year=2023)),
)
dr = date_range_from_datetime_range(dtr)
# before: dr.end == 2023-01-05 00:00:00 -> an event at 2023-01-05 09:30 is NOT in dr

The TypeScript original uses two helpers: toStartDate (midnight for a date-only value) for the start, and toStopDate (end-of-day, 23:59:59.999) for the stop. The Python port used the start-of-day conversion for both, so the end-of-day behavior was lost.

This adds a stop_datetime_from_date_time helper mirroring toStopDate: a date-only stop now covers the whole day (23:59:59.999999), while an explicit time is still honored as-is. Added tests for both cases; the full offline test suite passes.