MINOR: Prevent overflow in CreateTopics partition validation

[coderbruis]

This patch updates the CreateTopics partition count validation.

Changes:

• Update the validation comment to state that a request may create at most MAX_PARTITIONS_PER_BATCH partitions.
• Use a long counter to prevent integer overflow when accumulating partition counts from the request.
• Move the limit check inside the loop so that oversized requests fail as soon as the limit is exceeded.

Test:

• Add testTotalNumberOfPartitionsValidationDoesNotOverflow to cover a CreateTopics request large enough to overflow an int.