fix(chat): mock mode does not work in v2/chat/endpoint

What does this merge request do and why?

This MR fixes v2/chat/agent doesn't support mock mode (#676 - closed).

How to set up and validate locally

Application setting:

AIGW_MOCK_MODEL_RESPONSES=true

Request example:

curl -X 'POST' \
  'http://localhost:5052/v2/chat/agent' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -H 'x-gitlab-enabled-feature-flags: expanded_ai_logging' \
  -d '{
  "messages": [
    {
      "role": "user",
      "content": "How can I log output?",
      "additional_context": [
        { "id": "123", "category": "merge_request", "content": "something", "metadata": {"a": "b"}}
      ]
    },
    {"role": "assistant", "content": "Use print function"},
    {
      "role": "user",
      "content": "How can I log output?",
      "context": { "type": "issue", "content": "This is awesome issue" },
      "current_file": { "file_path": "main.py", "data": "print", "selected_code": true },
      "additional_context": [
        { "id": "456", "category": "file", "content": "helloworld", "metadata": {"c": "d"}}
      ]
    }
  ],
  "options": {
    "agent_scratchpad": {
      "agent_type": "react",
      "steps": []
    }
  }
}'

Response:

{"type": "unknown", "data": {"text": "mock"}}

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.