fix: handle SET options case-insensitively

[AysajanE]

Summary

• normalize supported SET option keywords before parsing them
• make lowercase options such as ex behave like their uppercase Redis equivalents
• cover lowercase expiration handling for both string and buffer reply variants

Checks

• ./node_modules/.bin/jest --config jest.config.js test/integration/commands/set.js --runInBand — current base: expected regression failure; patched: 34 passed in the clean network-off verifier
• npm test -- --runInBand — 1,471 source tests, 1,365 built integration tests, and 1,245 browser tests passed; Node and browser builds also passed in separate network-off validation
• targeted Prettier and ESLint checks passed

Fixes #416

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AI assistance was used. This change was reviewed by Northset, and I accept responsibility for this submission.

Verification

Northset proof-of-pass receipt M-1031
Contributor self-run; not maintainer verification.