Anthropic rejects requests whose system field is an empty string with
"system: text content blocks must be non-empty". The connection test
hardcoded an empty system, and regular completion/stream requests sent
an empty system_prompt when none was configured.
Add a claude_system_prompt() helper that substitutes a non-empty default
for empty or whitespace-only prompts, and apply it to all Claude request
sites. Covered by a unit test.
iiusky
57f1b6cedc