feat: add support for ioredis Cluster with Sharded Pub/Sub

[seta-chuang-0909]

Summary

This PR adds support for using the sharded adapter with ioredis Cluster, which was previously not possible due to ioredis#1759.

Starting with ioredis@5.9.0, the sharded Pub/Sub functionality has been fixed via ioredis#1956, enabling proper routing of SSUBSCRIBE/SPUBLISH commands in cluster mode.

Changes

New Features

• Added isIoRedisCluster() helper function in lib/util.ts to detect ioredis Cluster instances
• Added hasShardedSubscribers() helper function in lib/util.ts to check if the shardedSubscribers option is enabled
• Added validation in ShardedRedisAdapter constructor to ensure users enable the shardedSubscribers option when using ioredis Cluster

Documentation

• Updated README.md with usage instructions for ioredis Cluster with sharded Pub/Sub
• Added table of contents entry for the new section

Tests

• Enabled the previously skipped [sharded] ioredis cluster test
• Added shardedSubscribers: true option to the test configuration
• Added natMap configuration for Docker environment compatibility

Dependencies

• Updated ioredis dev dependency from ^5.3.2 to ^5.9.0

Usage

import { Cluster } from "ioredis";
import { Server } from "socket.io";
import { createShardedAdapter } from "@socket.io/redis-adapter";

const pubClient = new Cluster(
  [
    { host: "localhost", port: 7000 },
    { host: "localhost", port: 7001 },
    { host: "localhost", port: 7002 },
  ],
  {
    shardedSubscribers: true, // Required for sharded Pub/Sub
  }
);
const subClient = pubClient.duplicate();

const io = new Server({
  adapter: createShardedAdapter(pubClient, subClient)
});

io.listen(3000);

# Breaking Changes
None. This is a new feature that enables previously unsupported functionality.

# Related Issues
Fixes the limitation mentioned in the README: "it is not currently possible to use the sharded adapter with the ioredis package and a Redis cluster"
Reference: https://github.com/redis/ioredis/pull/1956

[darrachequesne]

Merged as c2d668b. Thanks a lot 👍