Performance: Scope llm index updates to actually modified documents (#13322)

* Perf: scope bulk_update_documents' LLM index refresh to the edited document ids instead of the whole library

* Perf: select_related/prefetch_related for the scoped LLM index batch

* Perf: select_related/prefetch_related for the full LLM index rebuild path

* Fix: address Copilot review findings on LLM index scoping

src/documents/tests/test_tasks.py

@@ -401,6 +401,10 @@ class TestAIIndex(DirectoriesMixin, TestCase):
                 "documents.tasks.update_llm_index",
             ) as update_llm_index,
         ):
-            tasks.bulk_update_documents([doc.pk for doc in docs])
+            doc_ids = [doc.pk for doc in docs]
+            tasks.bulk_update_documents(doc_ids)
             self.assertEqual(update_document_in_llm_index.apply_async.call_count, 0)
-            update_llm_index.assert_called_once()
+            update_llm_index.assert_called_once_with(
+                rebuild=False,
+                document_ids=doc_ids,
+            )