Function build_env_filter 

pub fn build_env_filter(level: Option<&str>) -> EnvFilter

Build an EnvFilter from an explicit level/directive spec (e.g. "debug" or "bhtune_cli=debug,bhtune_runtime=debug,sqlx=warn"), falling back to info if level is absent or fails to parse. Logging misconfiguration should degrade gracefully rather than stop a tune from running.