Copilot review on #661: empty base_url let the SDK fall back to
https://api.anthropic.com, sending compat-shaped requests to the
commercial API. The lane is local-only by definition, and the /v1-strip
edge case already established fail-loudly-over-silent-prod-retarget;
apply the same principle to the empty case. create_client raises an
actionable ValueError; the admin Detect path surfaces it as a clean
error string via probe_model_endpoint's existing handler.
Patrick Buckley
7ef04e576a