fix(captain): read OpenAI key from InstallationConfig in article search terms (#14915)

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).

---------

Co-authored-by: Sony Mathew <2040199+sony-mathew@users.noreply.github.com>
Co-authored-by: Sony Mathew <sony@chatwoot.com>

.env.example

@@ -272,9 +272,9 @@ AZURE_APP_SECRET=
 # ENABLE_SIDEKIQ_DEQUEUE_LOGGER=false
 
 
-# AI powered features
-## OpenAI key
-# OPENAI_API_KEY=
+# AI powered features (Captain)
+# The OpenAI API key and endpoint for Captain are not configured via .env.
+# Set them at Super Admin > App Configs > Captain (CAPTAIN_OPEN_AI_API_KEY, CAPTAIN_OPEN_AI_ENDPOINT).
 
 # Housekeeping/Performance related configurations
 # Set to true if you want to remove stale contact inboxes