Use nested Redis auth secrets when password key is absent

Grant Youngrequested to merge
gy-fix-redis-subqueue-auth into master

What does this MR do?

MR fixes a bug where configuring a separate auth secret for a nested Redis class (e.g. global.redis.cache.auth.secret) was silently ignored, causing the global Redis secret to be used instead.

Root cause: gitlab.redis.selectedMergedConfig sets redisMergedConfig to a direct reference (not a copy) of the named Redis sub-config map. When the sub-config had no password key, it would fall back to global.redis.auth — but because redisMergedConfig is a reference, this mutated global.redis.cache in-place, adding password = global.redis.auth. By the time gitlab.redis.configMerge ran to resolve which secret to mount, it found cache.password already set to the global auth secret and used that.

Fix: Both selectedMergedConfig and configMerge now check for a nested auth key on the sub-config (e.g. global.redis.cache.auth) before falling back to the global global.redis.auth. This matches the pattern users naturally follow, since the global config uses global.redis.auth.

Related issues

Closes #5167 (closed)

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