refactor(document): address review concerns from PR #660

- Restore JavaDoc on DocumentSearchResult.of() and .paged() factory methods
- Remove redundant null guards on @Builder.Default collections in toListItem()
- Map DocumentListItem fields explicitly in DocumentMultiSelect before cast
- Add DocumentListItem required fields to docFactory in spec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/DocumentMultiSelect.svelte

@@ -46,7 +46,11 @@ function handleInput() {
 			const res = await fetch(`/api/documents/search?q=${encodeURIComponent(searchTerm)}&size=10`);
 			if (res.ok) {
 				const body: { items: DocumentListItem[] } = await res.json();
-				const docs = body.items as unknown as Document[];
+				const docs = body.items.map((it) => ({
+					id: it.id,
+					title: it.title,
+					documentDate: it.documentDate
+				})) as unknown as Document[];
 				results = docs.filter((d) => !selectedDocuments.some((s) => s.id === d.id));
 			}
 		} catch {