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/lib/chat_gpt.rb

@@ -1,62 +0,0 @@
-class ChatGpt
-  def self.base_uri
-    'https://api.openai.com'
-  end
-
-  def initialize(context_sections = '')
-    @model = 'gpt-4o'
-    @messages = [system_message(context_sections)]
-  end
-
-  def generate_response(input, previous_messages = [], role = 'user')
-    @messages += previous_messages
-    @messages << { 'role': role, 'content': input } if input.present?
-
-    response = request_gpt
-    JSON.parse(response['choices'][0]['message']['content'].strip)
-  end
-
-  private
-
-  def system_message(context_sections)
-    {
-      'role': 'system',
-      'content': system_content(context_sections)
-    }
-  end
-
-  def system_content(context_sections)
-    <<~SYSTEM_PROMPT_MESSAGE
-      You are a very enthusiastic customer support representative who loves to help people.
-      Your answers will always be formatted in valid JSON hash, as shown below. Never respond in non JSON format.
-
-      ```
-      {
-       response: '' ,
-      context_ids: [ids],
-      }
-      ```
-
-      response: will be the next response to the conversation
-
-      context_ids: will be an array of unique context IDs that were used to generate the answer. choose top 3.
-
-      The answers will be generated using the information provided at the end of the prompt under the context sections. You will not respond outside the context of the information provided in context sections.
-
-      If the answer is not provided in context sections, Respond to the customer and ask whether they want to talk to another support agent . If they ask to Chat with another agent, return `conversation_handoff' as the response in JSON response
-
-      ----------------------------------
-      Context sections:
-      #{context_sections}
-    SYSTEM_PROMPT_MESSAGE
-  end
-
-  def request_gpt
-    headers = { 'Content-Type' => 'application/json', 'Authorization' => "Bearer #{ENV.fetch('OPENAI_API_KEY')}" }
-    body = { model: @model, messages: @messages, response_format: { type: 'json_object' } }.to_json
-    Rails.logger.info "Requesting Chat GPT with body: #{body}"
-    response = HTTParty.post("#{self.class.base_uri}/v1/chat/completions", headers: headers, body: body)
-    Rails.logger.info "Chat GPT response: #{response.body}"
-    JSON.parse(response.body)
-  end
-end