fix: prevent next_day panic on far-future start dates

[viirya]

Which issue does this PR close?

• Closes [Bug] next_day returns NULL for far-future start dates where Spark returns a value #23891.

Rationale for this change

datafusion-spark's next_day computed its result by adding the day offset to a chrono::NaiveDate:

Some(Date32Type::from_naive_date(
    date + Duration::days((7 - date.weekday().days_since(day_of_week)) as i64),
))

When the next occurrence of the requested weekday lands past chrono::NaiveDate::MAX (epoch day 95026236) — which happens for start dates in the last week of the representable Date32 range — the NaiveDate + TimeDelta add panics with NaiveDate + TimeDelta overflowed. (The linked issue reports NULL; the scalar path in fact panics.)

Spark's DateTimeUtils.getNextDateForDayOfWeek is pure integer arithmetic on the epoch day and always produces a value:

def getNextDateForDayOfWeek(startDay: Int, dayOfWeek: Int): Int = {
  startDay + 1 + ((dayOfWeek - 1 - startDay) % 7 + 7) % 7
}

What changes are included in this PR?

• Compute the result on the epoch day directly (days + delta, via checked_add) instead of building a NaiveDate for the result, so a next occurrence past NaiveDate::MAX returns a value instead of panicking. The weekday/offset logic is unchanged, so results for all in-range dates are identical.

Scope note: a start day beyond NaiveDate::MAX (> 95026236) still returns NULL, because the weekday is derived via NaiveDate. Extending full Int-range parity (computing the weekday arithmetically too) is a larger change left as a follow-up; this PR fixes the panic, which is the concrete reported harm.

Are these changes tested?

Yes.

• Unit test next_day_handles_far_future_start_dates covering the two cases from the issue (95026236/Mon, 95026230/Tue), each of which panicked before this change.
• SLT coverage in spark/datetime/next_day.slt asserting the same two cases (casting the Date32 result to Int32, since these dates are past the printable range).

Are there any user-facing changes?

next_day no longer panics for far-future start dates; it returns the correct epoch day. No API changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.05%. Comparing base (66c3840) to head (f310c59).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24194      +/-   ##
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+ Coverage   80.86%   81.05%   +0.18%     
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  Files        1101     1106       +5     
  Lines      375446   382289    +6843     
  Branches   375446   382289    +6843     
==========================================
+ Hits       303592   309848    +6256     
- Misses      53761    54123     +362     
- Partials    18093    18318     +225     

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[viirya]

Thanks @Jefffrey for review