Find ShipStation purchase orders that were never billed in FreshBooks

**Objective:** For purchase orders in ShipStation with status `{po-status}` (default all statuses), check whether the supplier exists as a FreshBooks vendor and whether a bill has been entered for the order.

**Step 1: Read purchase orders (ShipStation server)**

Call `shipstation-get-purchase-order`, setting status to `{po-status}` when it is not empty. Keep `order-number`, `reference-number`, `supplier-name`, `supplier-id`, `order-date` and `expected-delivery-date`. If you need the supplier's contact email, call `shipstation-get-supplier` and match on supplier-id.

**Step 2: Find the vendor (FreshBooks server)**

For each distinct supplier-name call `freshbooks-get-vendor` with vendor-name set to the supplier-name; if nothing comes back, retry with vendor-name.contains. Take the vendor `id`.

**Step 3: Find the bill (FreshBooks server)**

For each vendor found, call `freshbooks-get-bill` with vendor-id set to that id. A bill matches a purchase order when its bill-number equals the order-number or reference-number.

**Step 4: Report**

A table with one row per purchase order: order-number, supplier, vendor found (yes or no), bill found (yes or no), bill status and outstanding amount. Group the rows into: no vendor in FreshBooks, vendor but no bill, billed.

**Rules & Guidelines:**

* `shipstation-get-purchase-order` and `shipstation-get-supplier` are on the shipstation server; `freshbooks-get-vendor` and `freshbooks-get-bill` are on the freshbooks server.
* Read-only. Do not call `freshbooks-add-vendor`, `freshbooks-add-bill` or any other freshbooks write tool.

**Validation:**

* If several vendors match one supplier name, list them and mark that row as ambiguous rather than picking one.
* If no purchase orders come back, say so and stop.