address code review: assert page count in multi-page test, document fixes

- Lock in the exact expected page count (4) for the 60+60-row pagination
  test, which previously only checked the buffer was non-trivial. This is
  the scenario most likely to expose a footer/pagination regression.
- Add short comments explaining the footerY height-bound fix and the
  pdfPageCount() regex's coupling to pdfkit's serialization format.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

myteamwallet_backend/src/cashbox-export/cashbox-export.utils.ts

@@ -313,6 +313,12 @@ function addFooters(doc: PDFKit.PDFDocument, teamName: string): void {
   for (let i = range.start; i < range.start + range.count; i++) {
     doc.switchToPage(i);
     const footerY = doc.page.height - 25;
+    // footerY sits inside the reserved bottom margin (below pdfkit's page
+    // maxY()). Without an explicit `height`, pdfkit's LineWrapper measures
+    // overflow against the full-page maxY() and calls addPage() here on every
+    // iteration - silently appending blank trailing pages. Bounding the text
+    // to its own small box (well over the 8pt single-line height needed)
+    // keeps the overflow check local and stops that auto-pagination.
     doc
       .fontSize(8)
       .font('Helvetica')