Make AI Block Model Field Optional

[archie-ai-code-explain-pr-review[bot]]

• The "Model" field in the AI block should be made optional, as the prompt field and API key are the only required fields for creating an AI block.
• Update the frontend to handle the optional fields properly, including the "Model" field.
• This change will simplify the AI block configuration and make it more user-friendly.
• Provide a PR quality scorecard with specific points to address, including:
  • Ensure the frontend handles the optional "Model" field gracefully
  • Update the backend to support the optional "Model" field
  • Add relevant tests to cover the new functionality
• Include screenshots showing the current and expected behavior.
• Provide any necessary debugging information and potential solutions.