Hide fully-cartoned shipments on the admin order page (#3357)

[luwiee]

Summary

Closes #3357.

The admin order edit page renders a "Shipped package from 'X'" legend twice for fully-shipped orders: once from the Carton partial and once from the Shipment partial that the carton was created from. The reporter's screenshot shows the visual confusion this causes — admins see two rows that look like distinct packages but represent the same items.

Approach

The Shipment partial already filters its manifest to inventory units with carton_id IS NULL. When that filter returns an empty list (i.e., every inventory unit has been moved into a Carton), the partial has nothing meaningful left to show:

• no items to display in the manifest table,
• no "Ship" action to expose (the shipment is already shipped),
• no tracking — that authoritatively lives on the Carton.

So this PR skips rendering the Shipment partial entirely in that case. The Carton partial remains the single source of truth for fully-shipped packages, and in-progress shipments (with any non-cartoned units) render unchanged.

Test

Added a non-JS feature spec:

Shipments
  an order whose units have all been moved into a carton
    does not render the now-empty shipment alongside the carton

Verified the spec fails on main with the exact symptom reported in #3357 ("H... - Shipped package from 'NY Warehouse'" matching [data-hook='admin_shipment_form']) and passes with the fix applied. Existing shipment specs continue to pass.

Trade-offs / alternatives considered

• Filter at the call site (_form.html.erb) instead of inside the partial — rejected because it would either trigger N+1 (shipments.select { |s| s.inventory_units.where(carton_id: nil).any? }) or push a more complex SQL filter into the view. Skipping render at the partial boundary keeps the change local and avoids any query restructuring.
• A larger refactor along the lines of [RFC] Nest carton into shipment view #4319 (RFC: nest cartons under shipments, possibly extract cartons into their own admin tab) — out of scope for a bug fix; this PR is intentionally minimal so the visible duplication can ship without waiting on a product decision about cartons.

Out of scope

• The Carton partial still has its own quirks (e.g., it doesn't show special-instructions like the Shipment partial does) — left untouched.
• No i18n changes.