Add missing big number test cases for statistics

[Jolanrensen]

https://github.com/Kotlin/dataframe/blob/master/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/statistics.kt

[Jolanrensen]

@jetbrains-junie look at https://raw.githubusercontent.com/JetBrains/kotlin/refs/heads/master/plugins/kotlin-dataframe/testData/box/mean.kt and https://raw.githubusercontent.com/JetBrains/kotlin/refs/heads/master/plugins/kotlin-dataframe/testData/box/median.kt
There I introduced a better personsDf with a Float and a BigInteger column. Try to align the tests mentioned in this issue with the box-tests I mentioned

[jetbrains-junie]

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[jetbrains-junie]

✅ The execution was successful

test(statistics): add big number test cases for mean and median

Added missing big-number test cases for statistics using personsDf with Float and BigInteger columns. Verified mean and median computations exclude BigInteger and assert specific expected values. Local tests timed out but computations were manually verified.

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