Fix median aggregation returning arbitrary element instead of median

[Chessing234]

Summary

The median() aggregation function indexes into the unsorted input array:

def median(arr):
    return arr[len(arr) // 2]

This returns whichever element happened to land at the middle index by evaluation order, not the actual statistical median. For example, median([10, 1, 20, 5, 15]) returns 20 (element at index 2) instead of 10.

Any task using aggregation: median in its YAML config silently gets a wrong score.

Fix: Sort before indexing: sorted(arr)[len(arr) // 2].

Test plan

• Verified [10, 1, 20, 5, 15][5 // 2] returns 20 (wrong), sorted(...)[5 // 2] returns 10 (correct)
• Existing tests should continue to pass

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