fix(rust, python): fix result dtype in date_range(..., eager=True) if duration contains "1s1d"

[MarcoGorelli]

On the way towards #9019

closes #9665

ideally, to be rebased onto #8591

Sorry for all the tests, my hope is that extensive tests now will mean fewer regression tests in the future 🤞

---

Summary:

Instead of doing

if not eager:
    <use date_range_lazy>
else:
    <use date_range_eager>

I'm doing

result = <use date_range_lazy>
if not eager:
    return result
return F.select(result).explode().set_sorted()

In order for that to work, I've had to align the lazy and eager paths a bit more. For example, in the eager mode, currently, a date range of dates with sub-daily precision results in a date_range of datetimes:

polars/py-polars/polars/functions/range.py

Lines 575 to 580 in f1470c2

	if (
	start_is_date
	and end_is_date
	and not _interval_granularity(interval).endswith(("h", "m", "s"))
	):
	dt_range = dt_range.cast(Date)

That logic was not present in the lazy date range, so I've added it.

The extra .explode really makes the issue raised in #9019 glaringly obvious - but this is a first step towards resolving it!
Also - this started off as an attempt to refactor, the fact that it picked up (and ended up solving) #9665 was just a fortunate find along the way 😄

[ritchie46]

Also - this started off as an attempt to refactor, the fact that it picked up (and ended up solving) #9665 was just a fortunate find along the way smile

Finders luck! ^^