fix(redis sink): treat PUBLISH with 0 subscribers as success

[gl-pup]

Summary

The redis sink using data_type: channel enters an infinite retry loop when a PUBLISH command returns 0 (no subscribers currently connected).

Root cause

Redis PUBLISH returns the number of subscribers that received the message as an integer. When no subscribers are connected, Redis returns :0\r\n. redis-rs deserializes this integer as bool false into Vec<bool>, causing is_successful() to return false, which triggers should_retry_response to return Retry — even though the connection and Redis server are both healthy.

This is fundamentally different from List and SortedSet operations, where a 0 response would indicate a genuine failure (LPUSH/RPUSH always returns the new list length ≥ 1 on success).

Fix

Short-circuit is_successful() to true for DataType::Channel. A zero-subscriber publish is a valid transient state in pub/sub (the consumer may have momentarily dropped its subscription) and must not be treated as an error.

List and SortedSet behavior is unchanged.

Test plan

• Added redis_channel_publish_zero_subscribers_is_successful: verifies that is_successful() returns true for DataType::Channel when event_status contains false (0 subscribers)
• Added redis_list_and_sorted_set_still_check_event_status: verifies that List and SortedSet still treat false event status as failure
• All 8 existing redis sink unit tests continue to pass

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[tanji]

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[gl-pup]

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