feat: use captain endpoint config in legacy OpenAI base service (#12060)

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

enterprise/app/models/enterprise/concerns/article.rb

@@ -68,8 +68,16 @@ module Enterprise::Concerns::Article
     headers = { 'Content-Type' => 'application/json', 'Authorization' => "Bearer #{ENV.fetch('OPENAI_API_KEY', nil)}" }
     body = { model: 'gpt-4o', messages: messages, response_format: { type: 'json_object' } }.to_json
     Rails.logger.info "Requesting Chat GPT with body: #{body}"
-    response = HTTParty.post('https://api.openai.com/v1/chat/completions', headers: headers, body: body)
+    response = HTTParty.post(openai_api_url, headers: headers, body: body)
     Rails.logger.info "Chat GPT response: #{response.body}"
     JSON.parse(response.parsed_response['choices'][0]['message']['content'])['search_terms']
   end
+
+  private
+
+  def openai_api_url
+    endpoint = InstallationConfig.find_by(name: 'CAPTAIN_OPEN_AI_ENDPOINT')&.value || 'https://api.openai.com/'
+    endpoint = endpoint.chomp('/')
+    "#{endpoint}/v1/chat/completions"
+  end
 end