Function parallel_summary 

pub fn parallel_summary() -> impl Fn(&str) -> String

Creates a summary generator for parallel operation results.

The returned closure accepts a serialized JSON array of results and produces a JSON string containing:

• type: "parallel"
• totalCount: total number of elements
• successCount: number of successful elements
• failureCount: number of failed elements
• status: "completed" if all succeeded, "partial" if some failed, "failed" if all failed

Each array element is considered a failure if it is an object containing an "error" field. All other elements are counted as successes.