feat(cluster): add getNodeClientForKey for WATCH/MULTI/EXEC

[nkaradzhov]

Cluster client cannot expose WATCH because it needs connection-level state on a specific node. Previously users had to import cluster-key-slot, compute the slot, read cluster.slots, and call nodeClient manually.

Add getNodeClientForKey(key, isReadonly?) which resolves the key's slot and returns the connected node client (master, or a slot node when readonly), enabling optimistic-locking transactions in cluster mode.

Closes #3194

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
Additive public API and thin routing helper on existing slot logic; no changes to default command dispatch or auth.

Overview
Adds getNodeClientForKey(key, isReadonly?) on the cluster client so callers can run connection-scoped work (especially WATCH → MULTI/EXEC) on the node that owns a key’s slot, without manually using cluster-key-slot, cluster.slots, and nodeClient.

Routing matches existing cluster behavior: slot master by default, or a slot node (including replicas) when isReadonly is true. docs/clustering.md documents the WATCH transaction pattern and the same-slot key constraint. Integration tests cover master resolution, WATCH/MULTI/EXEC, and readonly node selection.

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