Update the langchain/langgraph `on_llm_start` hook to use a different `op`

[ericapisani]

At the moment, the opis set to gen_ai.pipeline which is a legacy value that we have deprecated (and only appears in the Python SDK).

We need to update this to something more appropriate, either a text completion or chat completion. Which thing we update to will be determined by the outcome of this RFC.

[linear-code]

PY-2143