Batch SSUBSCRIBE by Redis Cluster slot to avoid CROSSSLOT errors

[bellatoris]

…ered bursts

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Description of change

This PR changes how SSUBSCRIBE commands are issued in Redis Cluster mode.

Previously, each channel was subscribed individually in a loop, resulting in one SSUBSCRIBE per channel. In Redis Cluster, this increases the risk of CROSSSLOT errors and introduces unnecessary overhead when multiple channels belong to the same hash slot.

This update improves the logic by grouping channels by their Redis hash slot, and sending a single SSUBSCRIBE per slot group, subscribing to all relevant channels at once.

Motivation

• Redis Cluster requires that all keys in a multi-key command reside in the same hash slot.
  Subscribing to multiple channels across slots in a single command causes CROSSSLOT errors.
• The previous implementation avoided this by sending individual commands per channel, but this is inefficient.
• Batching subscriptions by slot reduces command overhead and aligns with Redis Cluster's intended usage pattern.

Changes

• Group channels by their hash slot (key_slot(self.encoder.encode(channel)))
• Issue one SSUBSCRIBE command per slot group (i.e., SSUBSCRIBE channel1 channel2 ...)

Example

channels_by_slot = defaultdict(list)
for channel in new_s_channels:
    slot = key_slot(self.encoder.encode(channel))
    channels_by_slot[slot].append(channel)

[daniel-lee-sb]

LGTM