[FEATURE]: Allow using Gemini Nano Banana Series model to generate the image.

[Eric-Guo]

Hoping the #40126 can be merged as my son like T-Rax, it's based on V2 branch.

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Allow opencode generate the image using Google Gemini below series model:

• Nano Banana
• Nano Banana 2
• Nano Banana 2 Preview
• Nano Banana 2 Lite
• Nano Banana Pro
• Nano Banana Pro Preview

In the desktop (in #40126)

And TUI (not yet PR):

[github-actions]

Thanks for updating the issue! It now meets our contributing guidelines. We'll take a look.