Fix Uncloused Client Error trigger when use GoogleGenAIChatGenerator in a AsyncPipeline

[raymondWai]

Describe the bug
When I try to use GoogleGenAIChatGenerator in a AsyncPipeline, It always show Unclosed client session error.

To Reproduce
Code snippet to trigger

Describe your environment (please complete the following information):

• OS: Ubuntu 24.04.3 LTS
• Haystack version: 2.18.1
• google-genai-haystack version: 2.1.2
• Integration version: 2.1.2