test(llm-observability): Account for LangGraph 0.3.29 changes

[Twixes]

The failure of posthog/test/ai/langchain/test_callbacks.py::test_graph_state on #218 is caused by the very recent LangGraph 0.3.29 release, which has changed the callbacks fired. Seemingly it's the langchain-ai/langgraph#4247 PR.

In concrete terms, LangGraph 0.3.29+ does not register ChannelWrite<...> spans, and doesn't capture __start__ unless it has a conditional edge.

This PR updates test_graph_state to reflect the current behavior of LangGraph.

[greptile-apps]

PR Summary

This PR updates the test_graph_state function to accommodate changes in LangGraph 0.3.29, which modified how callback spans are registered during execution.

• Updated test assertions to reflect that LangGraph 0.3.29+ no longer registers ChannelWrite<...> spans
• Simplified test to expect 5 spans plus a trace event instead of the previous 10 spans
• Adjusted parent-child relationship assertions to match the new behavior where __start__ is not captured unless it has a conditional edge
• Fixed test to align with changes from PR #4247 in the LangGraph repository

Greptile AI: This PR updates the test_graph_state function to accommodate changes in LangGraph 0.3.29, which modified how callback spans are registered during execution.

• Updated test assertions to reflect that LangGraph 0.3.29+ no longer registers ChannelWrite<...> spans
• Simplified test to expect 5 spans plus a trace event instead of the previous 10 spans
• Adjusted parent-child relationship assertions to match the new behavior where __start__ is not captured unless it has a conditional edge
• Fixed test to align with changes from PR #4247 in the LangGraph repository

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[skoob13]

✅