Support for redis-cluster

[chrisguidry]

[chrisguidry]

Looks like we've got a pretty fundamental blocker:

redis.exceptions.RedisClusterException: EVALSHA - all keys must map to the same key slot

[chrisguidry]

Progress Report: Redis Cluster Support

This branch has grown considerably and we've discovered several interconnected issues that would benefit from being broken into smaller, more focused pieces.

What's in the branch (17 commits)

Core infrastructure:

• redis+cluster:// URL scheme detection and normalization
• Hash-tagged key prefixes ({name}:queue vs name:queue) for slot consistency
• Cluster client caching via get_cluster_client() (has issues, see below)
• Cluster pub/sub via direct primary node connection
• create_result_storage() that tries to pass cluster client to py-key-value's RedisStore

Test infrastructure:

• tests/cluster/ directory with docker-compose for 6-node cluster (will remove)
• CI workflow for cluster testing

Issues Discovered

1. Cluster client caching doesn't play well with pytest-asyncio - Cached clients survive across tests but pytest-asyncio creates new event loops per test, causing "attached to different loop" errors and ResourceWarnings. We need caching for performance (without it, cluster mode is excruciatingly slow), but need to solve the test isolation problem.

2. py-key-value's RedisStore doesn't support Redis Cluster - When we tried passing a RedisCluster client to RedisStore(client=...), it doesn't work. The library uses Redis client APIs that don't exist on RedisCluster. We'll implement our own cluster-aware store.

Proposed Breakdown into Smaller PRs

PR 1: Core cluster URL handling (foundation)

• is_cluster_url(), normalize_cluster_url(), get_connection_kwargs()
• key_prefix() for hash tags
• Docket prefix and key() methods
• All key methods updated to use self.key() pattern
• No connection management changes - just URL detection and key formatting

PR 2: Cluster-aware connection helpers

• redis_connection() context manager (handles both modes)
• pubsub_connection() context manager
• publish_message() helper
• Docket redis(), _pubsub(), _publish() methods
• Cluster client caching with proper lifecycle management

PR 3: Cluster-aware result storage

• Implement our own ClusterKeyValueStore that works with RedisCluster
• Can share with py-key-value maintainer later as a contribution

PR 4: Test infrastructure for cluster mode

• conftest.py updates for REDIS_VERSION=X.Y-cluster pattern
• Cluster detection, 3-node container management, host networking
• Proper test isolation for cached cluster clients
• Cluster mode slots in like any other Redis version - no separate test directory
• CI matrix entry for cluster

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We'll break things down into these smaller pieces based on @zzstoatzz's open PR (#121).