refactor(llm): return schema.groovy first when backup graph data

[MrJs133]

1 file:

• hugegraph-llm/src/hugegraph_llm/utils/hugegraph_utils.py

2 points:

• Modified the format of the backup data (vertices.json, edges.json, and schema.json)
• Automatically select and return either schema.groovy or schema.json