fix: preserve state in DistinctMedianAccumulator::evaluate() for window frame queries

[kumarUjjawal]

Which issue does this PR close?

• Closes Accumulators which don't implement retract_batch can still exhibit buggy behaviour #19612.

Rationale for this change

The DistinctMedianAccumulator::evaluate() method was using std::mem::take() which consumed the internal state, causing subsequent calls to return incorrect results. This was the last remaining item from #19612

What changes are included in this PR?

1. Changed DistinctMedianAccumulator::evaluate() to use .iter() instead of std::mem::take() to preserve internal state across multiple calls
2. Added sqllogictest case for distinct median with window frames

Are these changes tested?

Yes, added a sqllogictest

Are there any user-facing changes?

No

[Jefffrey]

Thanks @kumarUjjawal