feat(client): add XNACK command with options and tests

[nkaradzhov]

Description

Describe your pull request here

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

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Note

Low Risk
Low risk: additive new command wiring plus tests; only touches the exported command registry and should not affect existing command behavior.

Overview
Adds client support for Redis 8.8 XNACK, exposing it as xNack/XNACK with support for SILENT/FAIL/FATAL modes, variadic IDs encoded via IDS <len>, and optional RETRYCOUNT/FORCE flags.

Includes unit tests for argument serialization and an integration test that exercises xNack against a real server (gated on Docker Redis >= 8.8), and wires the command into the central commands/index.ts export map.

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