…ow frame queries (apache#19887)

## Which issue does this PR close?

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- Closes apache#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
apache#19612

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## 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

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## Are these changes tested?

Yes, added a sqllogictest

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## Are there any user-facing changes?

No

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