Execute chat requests via new endpoint in AI Gateway

Problem

In Implement a ReAct prompt on AI Gateway (#452204 - closed) we're implementing on AI Gateway a new endpoint to send chat question. It's needed so we can filter chat tools on Gateway. With that we don't need to assemble the chat prompt on Rails monolith and we can simplify the code.

Proposal

1. Create a new SingleActionExecutor that will send requests to LLM via a new chat Gateway endpoint. This executor should implement the same decision loop like ZeroShot::Executor.
2. Current AiGateway::Client has hardcoded chat endpoint that should be changed.
3. The new executor should be behind a feature flag, so we can incrementally release it.

How it should look:

# ee/lib/gitlab/llm/completions/chat.rb

if Feature.enabled?(:chat_react_prompt_on_gateway_experiment)
            Gitlab::Llm::Chain::Agents::SingleAction::Executor.new(
              user_input: prompt_message.content,
              context: context,
              response_handler: response_handler,
              stream_response_handler: stream_response_handler
            ).execute
          else
            Gitlab::Llm::Chain::Agents::ZeroShot::Executor.new(
              user_input: prompt_message.content,
              tools: tools,
              context: context,
              response_handler: response_handler,
              stream_response_handler: stream_response_handler
            ).execute
          end

# single_action_executor

def execute
  MAX_ITERATIONS.times do
    request = execute_streamed_request

    return request[:final_answer] if request[:final_answer]

    answer = Answer.from(thought: request[:thought], action: [:action], tools: tools, context: context)

     options[:agent_scratchpad] << "\nThought: #{answer.suggestions}"
     options[:agent_scratchpad] << answer.content.to_s

     tool_class = answer.tool

     picked_tool_action(tool_class)

     tool = tool_class.new(
       context: context,
       options: {
         input: user_input,
         suggestions: options[:agent_scratchpad]
       },
     stream_response_handler: stream_response_handler
    )

     tool_answer = tool.execute

     return tool_answer if tool_answer.is_final?

     options[:agent_scratchpad] << "Observation: #{tool_answer.content}\n"
   end

   Answer.default_final_answer(context: context)
 end

Note: there is a lot of opportunities for refactoring and reusing the code.