[SPARK-59459][CORE] Extract a helper for the HashPartitioner array-key check in PairRDDFunctions

[uros-b]

What changes were proposed in this pull request?

PairRDDFunctions repeats the same guard -- throw hashPartitionerCannotPartitionArrayKeyError() when the partitioner is a HashPartitioner and the key type is an array -- in four places: partitionBy and the three cogroup overloads. This extracts it into a single private helper, failOnHashPartitionerWithArrayKey(partitioner), and calls it from those four sites.

Notes:

• The partitionBy site wrote the condition in the opposite operand order (keyClass.isArray && partitioner.isInstanceOf[HashPartitioner]). Both operands are side-effect-free, so the helper's order is equivalent.
• The nested check in combineByKeyWithClassTag -- inside a broader keyClass.isArray block that also guards map-side combining -- is intentionally left as-is, since it is a different shape.

Why are the changes needed?

The guard was duplicated verbatim; consolidating it removes the repetition and keeps the array-key / HashPartitioner rule in one place. Behavior is unchanged.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing tests; build/sbt core/compile passes. This is a behavior-preserving refactor.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[uros-b]

Waiting for CI here.