feat: Migrated to support LangGraph + LangChain v 1.0

[martinmarroyo]

This PR completes the migration from supporting LangGraph + LangChain < 1.0 to LangGraph + LangChain >= 1.0. It fully supports the latest stable release of these frameworks and adds some new features:

• Users no longer need to wrap their LLM invocations in their nodes with the execute method. Instead, our graph overloads the invoke, ainvoke, stream, and astream methods to capture token usage. This enables a more seamless experience where users now only have to focus on instrumenting the graph itself
• We emit an additional span that identifies the overall graph

These features have been tested locally and confirmed to work.