Encode Redis Sentinel passwords inside runtime ERB

What does this MR do?

Encode Redis Sentinel passwords after reading the secret, so quotes, backslashes, and embedded newlines survive YAML parsing. The shared helper now uses to_json inside ERB, and the Rails, gitlab-exporter, and Mailroom consumers no longer add outer quotes.

The regression renders the runtime YAML for webservice, sidekiq, toolbox, exporter, incoming email, and Service Desk. All eight password cases pass; the quote, backslash, and newline cases failed before the fix. The full Redis, exporter, Mailroom, and regression suites pass all 99 examples with seed 17. Ruby lint passes.

AI assistance: prepared with OpenAI Codex and tested locally.

Local checks used Ruby 3.2.3, Psych 5.5.0, Helm 4.2.3, and Gomplate 5.0.0. Local dependency adjustments are kept outside the patch. Cluster validation remains pending because the local Docker daemon is unavailable; these checks cover Helm rendering, runtime ERB evaluation, and YAML parsing. The supported configuration and secret mounts are unchanged, so no documentation update is needed. No Omnibus equivalent is proposed here because this fixes the shared Helm ERB helper and its consumers.

Related issues

Closes #6697 (closed)

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