Function group_findings_by_engine 

pub fn group_findings_by_engine(
    findings: Vec<ExternalFinding>,
) -> HashMap<String, Vec<ExternalFinding>>

Group a flat list of findings by engine name.

Returns a HashMap keyed on finding.engine, with values being all findings from that engine in input order. This is the grouping step that precedes ExternalStore::replace_engine: the caller parses one or more SARIF files into a flat Vec<ExternalFinding>, groups by engine, then calls replace_engine once per engine — ensuring that two SARIF files from the same engine are combined, not overwritten.