feat: HRANDFIELD and ZRANDMEMBER

[kacy]

summary

• HRANDFIELD: returns random field(s) from a hash; positive count = distinct, negative = allow duplicates; WITHVALUES returns interleaved field-value pairs
• ZRANDMEMBER: same semantics for sorted sets; WITHSCORES returns interleaved member-score pairs
• both commands follow the same positive/negative count convention as SRANDMEMBER

tested

• 7 unit tests in keyspace/hash.rs covering all count modes, WITHVALUES, missing key, wrong type
• 7 unit tests in keyspace/zset.rs covering all count modes, WITHSCORES, missing key, wrong type
• 9 integration tests: single return, positive/negative count, WITHVALUES/WITHSCORES, missing key (both commands)
• cargo clippy -D warnings clean

design notes

• no AOF record needed — both are pure read commands
• count=None returns a single bulk string (not a one-element array), matching redis spec
• count=0 returns an empty array, consistent with SRANDMEMBER behavior