generate_article_search_terms still pulled ENV['OPENAI_API_KEY'], left
over from before the Jan 2025 Captain migration moved the key into
InstallationConfig as CAPTAIN_OPEN_AI_API_KEY. Every other Captain LLM
call site got updated then; this one (used by Portal::ArticleIndexingJob
for help center article embedding search terms) didn't, so it sent a
blank bearer token unless you also happened to have the old env var set.
Also drops the stale OPENAI_API_KEY line from .env.example and points to
where the key actually lives now (Super Admin > App Configs > Captain).
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Co-authored-by: Sony Mathew <2040199+sony-mathew@users.noreply.github.com>
Co-authored-by: Sony Mathew <sony@chatwoot.com>
