feat: improve redis dedupe failover handling [poc]

[turip]

Summary

• Add an explicit sink.dedupeWriteTimeout around the post-persist Redis dedupe write path.
• Keep dedupe writes retrying until that timeout expires instead of stopping after a fixed attempt count.
• Recreate the Redis dedupe client/pool after write-path errors that indicate a stale connection or a read-only primary connection.
• Expose Redis client pool and timeout settings, with a small default pool size of 3.
• Add focused Redis dedupe tests for reconnect and non-reconnect paths.

Flow-level rationale

The sink flow persists a batch to storage, stores Kafka offsets, and then writes dedupe keys to Redis. At that point the worker is in the consistency-sensitive part of the flow: if Redis dedupe writes fail, future replays can accept duplicate events even though the batch was already persisted.

Before this change, the dedupe write retried with implicit retry defaults. During Redis primary failover, go-redis can remove only the checked-out bad connection while other stale connections remain in the pool. A fixed retry count can therefore be consumed by stale pooled connections before a fresh connection dials and resolves the writable endpoint again.

This change keeps the read-side uniqueness checks fail-fast, but makes the post-persist dedupe write path more resilient. The retry loop is bounded by dedupeWriteTimeout, and reconnect-worthy write errors now swap the Redis client so the next retry starts with a fresh pool and fresh endpoint resolution.

Validation

• go test ./openmeter/dedupe/redisdedupe ./app/config ./pkg/redis ./openmeter/sink ./app/common

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