Function log_completion 

pub fn log_completion(
    log_level: LogLevel,
    function_name: &str,
    kind: &str,
    input: &Value,
    duration: Duration,
    success: bool,
    error_msg: Option<&str>,
)

Emit a structured log line for a function execution.

Failures always log at error regardless of the configured level so that operator dashboards never miss a failed call. Successes log at the configured level. The input payload is logged at debug so callers can opt out of payload visibility by raising the global filter.