Stabilize RP route status parsing in flight plan headers

[Copilot]

FlightPlanUtils.parseHeader handled RP differently from the other route status prefixes: it set raw.route_status to 'RP' and then overwrote it with the full header. This made raw.route_status inconsistent and dropped the opportunity to parse inline route content carried in RP headers.

• Route status normalization

  • Keep raw.route_status as the enum-like status code 'RP', consistent with RF, RI, RM, and RS.
  • Remove the branch-local overwrite with the full header string.

• Inline route parsing

  • When an RP header contains route payload after the prefix, parse it as route content instead of storing it in route_status.
  • Align RP behavior with the existing RF branch.

• Regression coverage

  • Add a focused unit test for parseHeader covering an RP header with embedded route data, asserting both stable route_status and parsed waypoints.

} else if (header.startsWith('RP')) {
  decodeResult.raw.route_status = 'RP';
  decodeResult.formatted.items.push({
    type: 'status',
    code: 'ROUTE_STATUS',
    label: 'Route Status',
    value: 'Route Planned',
  });
  if (header.length > 2) {
    addRoute(decodeResult, header.substring(2));
  }
}