fix: median retract logic for sliding window frames (#21300)

## Which issue does this PR close?

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- Closes #21299.

## Rationale for this change

`median` could produce incorrect results for sliding window frames
because `MedianAccumulator::retract_batch` could skip values after
`swap_remove(i)`.

When an element was removed, the last element was moved into index `i`,
but the loop still incremented `i`. That meant the swapped-in value was
never checked, so not all requested values were retracted.

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## What changes are included in this PR?

- fix `retract_batch` so the index is only advanced when no value is
removed
- add a slt case for `median` over a sliding `range` window

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## Are these changes tested?

Yes.

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## Are there any user-facing changes?

No.

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Co-authored-by: Jeffrey Vo <jeffrey.vo.australia@gmail.com>

Author: lyne7-sc

datafusion/functions-aggregate/src/median.rs

@@ -314,8 +314,9 @@ impl<T: ArrowNumericType> Accumulator for MedianAccumulator<T> {
                         break;
                     }
                 }
+            } else {
+                i += 1;
             }
-            i += 1;
         }
         Ok(())
     }

datafusion/sqllogictest/test_files/aggregate.slt

@@ -1223,6 +1223,22 @@ ORDER BY tags, timestamp;
 4 tag2 90 75 80 95
 5 tag2 100 80 80 100
 
+# Test median on a sliding RANGE window with repeated values.
+query RR
+SELECT
+    x,
+    median(x) OVER (
+        ORDER BY x
+        RANGE BETWEEN 1 PRECEDING AND CURRENT ROW
+    ) AS median_val
+FROM (VALUES (1.0), (1.0), (2.0), (5.0)) AS t(x)
+ORDER BY x;
+----
+1 1
+1 1
+2 1
+5 5
+
 ###########
 # Issue #19612: Test that percentile_cont produces correct results
 # in window frame queries. Previously percentile_cont consumed its internal state