(v11) Drop gen_ai.create_agent spans from LangGraph instrumentation #22335

The LangGraph instrumentation still emits gen_ai.create_agent spans, but these are no longer relevant for us. This is a leftover we should clean up for v11 so we don't carry it into the major.

Both instrumentation paths emit the span today and both need to be handled:

• OTel path: packages/node/src/integrations/tracing/langgraph/
• Orchestrion (diagnostics-channel) path: packages/server-utils/src/integrations/tracing-channel/langgraph.ts
• Shared span options live in packages/core/src/tracing/langgraph/index.ts (_INTERNAL_getLangGraphCreateAgentSpanOptions, instrumentStateGraphCompile)

This was parked from #22268: #22268 (comment)