Update fake models to return different responses

What does this MR do and why?

This MR replaces the hard-coded response with one included the original input prompt.

• Before: fake code suggestion from PaLM Text
• After: Fake response for: <input prompt>

The requirements are as mentioned in #404 (closed)

Closes #404 (closed)

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure AIGW_USE_FAKE_MODELS=false in .env file
3. Run the AI Gateway.

   poetry run ai_gateway

4. Send a cURL request to the service.

   curl --request POST \
     --url http://ai-gateway.gdk.test:5052/v2/code/generations \
     --header 'Content-Type: application/json' \
     --header 'X-Gitlab-Authentication-Type: oidc' \
     --data '{
     "current_file": {
       "file_name": "hello.go",
       "content_above_cursor": "",
       "content_below_cursor": ""
     },
     "prompt_version": 2,
     "prompt": "// Generate a function to print hello world\nfunc print",
     "model_provider": "anthropic",
     "model_name": "claude-2.1"
   }'

5. Confirm the response is correct.

   {
     "id": "id",
     "model": {
       "engine": "fake-palm-engine",
       "name": "fake-palm-model",
       "lang": "go"
     },
     "experiments": [],
     "object": "text_completion",
     "created": 1710414858,
     "choices": [
       {
         "text": "Fake response for: // Generate a function to print hello world\nfunc print",
         "index": 0,
         "finish_reason": "length"
       }
     ]
   }