The client previously fell back to a hardcoded project-level API key
when the caller passed an empty string. Once committed the key cannot
be rotated without a rebuild, and the silent fallback meant that
misconfiguration produced unattributed traffic against a shared key.
Remove the default. Clients constructed with an empty key now return
ErrMissingAPIKey from every API call so operators see the
configuration problem immediately.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
