fetch_langsmith_project_url

Fetch the LangSmith project URL via the LangSmith client.

Results are cached at module level so repeated calls do not make additional network requests. Failed lookups are also cached to avoid retries.

This is a blocking network call on the first invocation. In async contexts, run it in a thread (e.g. via asyncio.to_thread).

Returns None (with a debug log) on any expected failure: missing langsmith package, network errors, invalid project names, or client initialization issues.