This PR migrates the legacy OpenAI integration (where users provide
their own API keys) from using hardcoded `https://api.openai.com`
endpoints to use the configurable `CAPTAIN_OPEN_AI_ENDPOINT` from the
captain configuration. This ensures consistency across all OpenAI
integrations in the platform.
## Changes
- Updated `lib/integrations/openai_base_service.rb` to use captain
endpoint config
- Updated `enterprise/app/models/enterprise/concerns/article.rb` to use
captain endpoint config
- Removed unused `enterprise/lib/chat_gpt.rb` class
- Added tests for endpoint configuration behavior
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