Update operate.py

[FeHuynhVI]

Standardized Prompt for Mix Mode

Description
This pull request introduces a standardized prompt for the "Mix Mode" in the LightRAG system. The prompt integrates both Knowledge Graph (KG) and Document Chunks (DC) data sources to support hybrid retrieval and generation, enhancing the assistant's ability to provide accurate and contextually relevant responses.

Checklist

• [x ] Changes tested locally
• [x ] Code reviewed
• Documentation updated (if necessary)
• Unit tests added (if applicable)

Additional Notes

[Add any additional notes or context for the reviewer(s).]

[LarFii]

Thanks for sharing! The use of the or operator to assign sys_prompt assumes that both system_prompt and PROMPTS["mix_rag_response"] require the same format arguments. Otherwise the unconditional call to format will result in an error.