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

## 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.

Start with RedisSinkTest and the embedded-redis test dependency; trace how redis.embedded.RedisServer is created and configured, then compare the container-based setup used by other connectors. Done means RedisSinkTest runs end to end against an official Redis image, with the existing coverage preserved and container-specific configurations considered within scope.

Written by the indexing model from the issue text.