Enrich Sorted Set operations

[imperugo]

Summary

Add missing Sorted Set operations critical for leaderboards, priority queues, and ranking.

Missing Methods

• `SortedSetCombineAsync` / `SortedSetCombineAndStoreAsync` — union/intersect/diff
• `SortedSetPopAsync` — pop min/max elements
• `SortedSetIncrementAsync` / `SortedSetDecrementAsync` — score manipulation (ZINCRBY)
• `SortedSetRandomMemberAsync` — random sampling
• `SortedSetLengthAsync` — cardinality
• `SortedSetScoreAsync` / `SortedSetScoresAsync` — get score(s)
• `SortedSetRankAsync` — get rank of member
• `SortedSetScanAsync` — iterate members

All available in SE.Redis 2.12. Thin wrappers with serialization.