Move Geotab devices to a new plan, checked and confirmed

**Objective:** Move device(s) {device-serials} to device plan ID {new-plan-id}, applying promo code {promo-code} if given. These tools change live MyAdmin data. Never call a write tool before the user has explicitly confirmed the exact values in this conversation.

**Step 1: Current state**

For each serial, call `geotab-myadmin-get-device-contract` with device-serial = the serial. Record current device plan, device-plan-id, database and terminated flag. Drop any serial that is not found or is terminated, and say why.

**Step 2: Check for conflicts**

For each remaining serial, call `geotab-myadmin-get-device-contract-auto-request` with device-serial = the serial and active = true. A pending request may override this change; list any you find.

**Step 3: Confirm**

Show a table: serial, current plan, current plan ID, new plan ID, promo code. Ask the user to confirm. Stop if they do not.

**Step 4: Change**

* One serial: call `geotab-myadmin-change-device-billing-plan` once with serial-no, device-plan-id = {new-plan-id}, promo-code if given.
* Several: call `geotab-myadmin-change-device-billing-plan-bulk` once with serial-nos as a JSON array, device-plan-id = {new-plan-id}.

**Step 5: Verify**

Call `geotab-myadmin-get-device-contract` again for each serial and report the device plan now shown. Report the tool's result as returned.

**Rules & Guidelines:**

* Call the write tool once. If it fails, report the error and stop; do not retry with different values.
* If a serial already has device-plan-id = {new-plan-id}, skip it.

**Validation:**

* If no serial survives Step 1, stop without writing.