Fix Qwen model prompt formatting:

- Add Qwen system message to system_messages dictionary
- Refactor get_system_prompt to use format_chat_history for consistent
formatting
- Fix empty assistant responses and extra newlines in Qwen chat format
- Fix following error message
```
[2025-03-24 14:30:04] | ERROR | aiohttp.server.log_exception | web_protocol.py:451 | Error handling request from 127.0.0.1
Traceback (most recent call last):
  File "C:\Users\kalin\miniconda3\envs\gaiaenv3\lib\site-packages\aiohttp\web_protocol.py", line 480, in _handle_request
    resp = await request_handler(request)
  File "C:\Users\kalin\miniconda3\envs\gaiaenv3\lib\site-packages\aiohttp\web_app.py", line 569, in _handle
    return await handler(request)
  File "C:\Users\kalin\Work\gaia3\src\gaia\agents\agent.py", line 204, in _on_prompt_received
    response = self.prompt_received(data["prompt"])
  File "C:\Users\kalin\Work\gaia3\src\gaia\agents\Chaty\app.py", line 41, in prompt_received
    response = self.prompt_llm(prompt)
  File "C:\Users\kalin\Work\gaia3\src\gaia\agents\Chaty\app.py", line 31, in prompt_llm
    prompt = Prompts.get_system_prompt(
  File "C:\Users\kalin\Work\gaia3\src\gaia\agents\Chaty\prompts.py", line 264, in get_system_prompt
    system_message = cls.system_messages[model_type]
KeyError: 'qwen'
```

closes #24