feat(store) : add new model and prompt in image generation

[Abhi1992002]

Update Marketplace Image generation Prompt and Model

Changes:

• Updated the image generation prompt for Marketplace to better highlight agent functionality:

  Create a visually engaging app store thumbnail for the AI agent that highlights what it does in a clear and captivating way:
  - **Name**: {agent.name}  
  - **Description**: {agent.description}  
  Focus on showcasing its core functionality with an appealing design.
  

• Changed the model to black-forest-labs/flux-1.1-pro for improved results.

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