fix(set): return scalar for SRANDMEMBER without count

[raghav2005]

Summary

• return a Redis-compatible bulk string for SRANDMEMBER key
• return nil for a missing key when count is omitted
• preserve collection replies for SRANDMEMBER key count
• add regression coverage for scalar, counted, and missing-key reply types

Closes #2610.

Testing

• ./x.py build build-docker --unittest --ninja -j 4
• ./x.py check format
• ./x.py check tidy build-docker -j 4
• ./x.py check golangci-lint
• ./x.py test go build-docker
• /workspace/build-docker/unittest (623 passed, 1 skipped)
• /workspace/build-docker/unittest --gtest_filter=RedisSetTest.TakeWithoutPop

[github-actions]

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