Refactor parse_cluster_slots and add slot range validation

[bjosv]

Restructure parse_cluster_slots to improve readability and add early validation of slot ranges received from the server.

Changes:

• Extract node entry parsing into parseClusterSlotsNodeEntry helper, eliminating duplicated validation logic for primary and replica nodes.
• Flatten the deeply nested inner loop: parse slot range from elements 0 and 1 directly, handle the primary at element 2, then iterate replicas starting at element 3.
• Validate slot ranges immediately (reject start > end and end >= VALKEYCLUSTER_SLOTS) instead of letting invalid values propagate until caught later in updateNodesAndSlotmap.
• Simplify error messages.
• Add unit test for invalid slot ranges.