Issue #3584031: RagTool::execute() should expose structured per-result data

execute() only ever built a flattened text string for the LLM; $this->results was never populated, so getStructuredOutput() always returned an empty results array. Populate it with entity type, entity id, langcode, score, and chunk text per surviving result, parsed from drupal_entity_id the same way SearchApiAiSearchBackend::checkEntityAccess() already does.

Closes #3584031