Function build_default 

pub fn build_default() -> SummarizerResult<Arc<dyn Summarizer>>

Build the default summarizer from environment.

1. CEL_SUMMARIZER_PROVIDER=anthropic (or unset) → try Anthropic; fall back to Ollama when the API key is missing or construction fails.
2. CEL_SUMMARIZER_PROVIDER=ollama → Ollama directly.