[improve][test] Redis sink: optionally migrate embedded Redis to a Testcontainers image

[david-streamlio]

Current state (correction)

redis already has integration coverage: RedisSinkTest runs RedisSink end-to-end against a real embedded Redis server (redis.embedded.RedisServer, the embedded-redis test dependency). This is genuine functional coverage, not a mocked unit test — an earlier triage note incorrectly listed this as a gap.

Enhancement (optional, not a gap)

Migrate the embedded server to a Testcontainers-based redis image so the module:

• matches the container-based approach used across the other connectors,
• runs against an official Redis build rather than a bundled binary, and
• can exercise auth / TLS / cluster configurations the embedded server can't.

This is a test-infrastructure improvement, not a coverage gap.