feat(bookstack-api-m6n): implement pagination iterator with iter.Seq2

Add generic listAll helper using Go 1.23+ iter.Seq2 for memory-efficient
pagination. Implement ListAll() on BooksService and PagesService.
Tests cover multi-page iteration, early break, errors, and empty results.

iterator_test.go

@@ -0,0 +1,103 @@
+package bookstack
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"testing"
+)
+
+func TestBooksService_ListAll(t *testing.T) {
+	callCount := 0
+	c := testClient(t, func(w http.ResponseWriter, r *http.Request) {
+		callCount++
+		offset := r.URL.Query().Get("offset")
+		switch offset {
+		case "0":
+			json.NewEncoder(w).Encode(map[string]any{
+				"data":  []map[string]any{{"id": 1, "name": "Book 1"}, {"id": 2, "name": "Book 2"}},
+				"total": 3,
+			})
+		case "2":
+			json.NewEncoder(w).Encode(map[string]any{
+				"data":  []map[string]any{{"id": 3, "name": "Book 3"}},
+				"total": 3,
+			})
+		default:
+			t.Errorf("unexpected offset %q", offset)
+		}
+	})
+
+	var books []Book
+	for book, err := range c.Books.ListAll(context.Background()) {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		books = append(books, book)
+	}
+
+	if len(books) != 3 {
+		t.Fatalf("got %d books, want 3", len(books))
+	}
+	if callCount != 2 {
+		t.Errorf("API called %d times, want 2", callCount)
+	}
+}
+
+func TestBooksService_ListAll_EarlyBreak(t *testing.T) {
+	c := testClient(t, func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(map[string]any{
+			"data":  []map[string]any{{"id": 1}, {"id": 2}, {"id": 3}},
+			"total": 100,
+		})
+	})
+
+	count := 0
+	for _, err := range c.Books.ListAll(context.Background()) {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		count++
+		if count == 2 {
+			break
+		}
+	}
+	if count != 2 {
+		t.Errorf("got %d items, want 2 (early break)", count)
+	}
+}
+
+func TestBooksService_ListAll_Error(t *testing.T) {
+	c := testClient(t, func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte("server error"))
+	})
+
+	for _, err := range c.Books.ListAll(context.Background()) {
+		if err == nil {
+			t.Fatal("expected error")
+		}
+		return // got the error, done
+	}
+	t.Fatal("iterator should have yielded an error")
+}
+
+func TestBooksService_ListAll_Empty(t *testing.T) {
+	c := testClient(t, func(w http.ResponseWriter, r *http.Request) {
+		json.NewEncoder(w).Encode(map[string]any{
+			"data":  []any{},
+			"total": 0,
+		})
+	})
+
+	count := 0
+	for _, err := range c.Books.ListAll(context.Background()) {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		count++
+	}
+	if count != 0 {
+		t.Errorf("got %d items, want 0", count)
+	}
+}