When no Anthropic API credential is available, the proxy now falls back
to OpenRouter before trying the Claude CLI. The new streamViaOpenRouterFallback
function converts OpenRouter's OpenAI-format SSE to Anthropic-format SSE
(content_block_delta with text_delta) so the frontend's Claude provider
can parse it correctly.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>