Get the agent ID for a LangSmith tracer.
Experimental: in beta and enabled per workspace. A workspace without agent addressing rejects the runs, so tracing is lost rather than falling back to a project.
Must be 1 to 255 characters.
Read from LANGSMITH_AGENT_ID only -- there is no LANGCHAIN_ alias,
so that the newer namespace is the only one to learn. This is an agent
identifier, not a credential: the server resolves the agent by this ID and
creates one if it doesn't exist yet. When unset the run is addressed by
project instead.
Read once per process and cached; call .cache_clear() to re-read.
