fix(sandbox): implement Redis set ops and raw command/pipeline instead of raising

[MikaAK]

Summary

Cache.Sandbox raised "Not Implemented" from smembers/3, sadd/4, command/3, command!/3, pipeline/3 and pipeline!/3.

Under sandbox?: Mix.env() === :test the use Cache wrappers delegate to Cache.Sandbox, so dialyzer (running in test) has been correctly inferring that every Redis cache module's injected command/1,2, command!/1,2, pipeline/1,2, pipeline!/1,2, sadd/2,3, smembers/2 have no local return — 11 no_return warnings per Redis cache module, forcing a :no_return ignore entry per module in every consuming app.

• sadd/smembers — real in-memory sets (MapSet under the key), Redis semantics: SADD returns the new-member count, SMEMBERS of a missing key is [].
• command/pipeline — PING, GET, EXISTS, DEL mapped onto the sandbox map; anything else returns {:error, %ErrorMessage{code: :not_implemented}} — the same error shape the Redis adapter produces on failure — instead of raising. Raw GET returns the stored binary undecoded, exactly as Redis would (the Redis-backed sandbox term-encodes on put). pipeline runs commands in order and halts on the first error.
• Bang variants raise only on that error branch, so they have a real success typing too.

Verification

• New test/cache/redis_sandbox_test.exs (11 tests, sandbox only, no Redis needed) — green.
• Dialyzer on a sandbox?: true Redis cache module: main produces the 11 no_return warnings above; this branch produces 0.
• Consuming umbrella (cheddar_flow_ex, 6 Redis cache modules): with this branch as a path dep and all six :no_return ignore entries deleted, dialyzer goes 174 → 108 total warnings, all remaining ones already skipped, 0 unnecessary skips.
• Full suite: the 13 pre-existing real-Redis (RedisJSONTest/RedisHashTest under parallel contention) failures are unchanged from main; everything else green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.87%. Comparing base (3c6c060) to head (16bbe13).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
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  Coverage   83.87%   83.87%           
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  Files          24       24           
  Lines         707      707           
=======================================
  Hits          593      593           
  Misses        114      114           

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