LangGraph — Align with gen_ai RFC #153

[andreiborza]

Align the LangGraph integration with RFC #153.

Attribute renames

• gen_ai.system → gen_ai.provider.name

Required attributes

• gen_ai.operation.name set on all spans
• gen_ai.provider.name set on all AI Client spans

Span changes

• Remove gen_ai.create_agent span type
• Ensure proper span hierarchy: invoke_agent → AI Client → execute_tool → AI Client

Message format

• content → parts in gen_ai.input.messages and gen_ai.output.messages

[linear-code]

JS-2328