feat(wren-ai-service): update the system prompt for table columns selection to provide clear instructions and expected JSON format.

[paopa]

This PR updates the system prompt used for table columns selection in the AI service. The changes aim to provide clearer instructions and a more structured JSON format for the AI's response.

Key Changes

1. Refined the task description to be more concise and focused.
2. Added a structured "INSTRUCTIONS" section with step-by-step guidance.
3. Updated the "FINAL ANSWER FORMAT" section to require a more detailed JSON structure:
   • Each table now includes a list of reasons (chain_of_thought_reasoning) for column selection.
   • The reasoning is now presented as a list of individual points rather than a single string.
4. Added an "ADDITIONAL NOTES" section to emphasize important requirements.
5. Removed unnecessary motivational language and focused on clear, professional instructions.

Expected Impact

• Improved clarity and structure in the AI's responses.
• More detailed reasoning for column selection, aiding in transparency and accuracy.

[cyyeh]

LGTM