Reuse the order's line items when building a shipping manifest

[mamhoff]

Summary

Spree::ShippingManifest builds its items from inventory units, and reaches the line
item through unit.line_item. Since Rails has no identity map, that returns a second
instance of a row the caller almost always already holds in memory as part of
order.line_items.

Two consequences:

1. Stale data. Adjustments recalculated on the order's line items but not yet
   persisted are invisible to the manifest, which reports the values as they were on
   disk. This is the bug that motivated the change.
2. A query per line item, on a path that is usually rendered once per shipment.

This adds an optional order: keyword to ShippingManifest#initialize. When given, and
when that order's line items are already loaded, the manifest resolves each item's line
item from the order instead of from the inventory unit. It falls back to unit.line_item
otherwise, so behaviour is unchanged for every existing caller.

Implementation notes

• Query-neutral by construction. The reuse is skipped entirely unless
  order.association(:line_items).loaded?. It is an optimization for callers that already
  have the line items, never a reason to issue a query.
• For the same reason, Shipment#manifest passes order: (order if association(:order).loaded?) rather than order: order — it must not trigger a load of
  the order. In the common path (order.shipments) the association is already populated
  via inverse_of, so this costs nothing.
• Nothing is mutated: the lookup happens where the manifest item is constructed, so no
  association targets are reassigned and no inventory unit is touched.
• for_order propagates the order, which is how Carton#manifest_for_order picks this
  up. Carton#manifest is deliberately left alone — a carton can span multiple orders,
  so there is no single order to pass.
• nil line_item_id (the association is optional: true) and units belonging to a
  different order both fall through to the existing behaviour via Hash#fetch with a
  block.
• The group_by(&:variant_id) / group_by(&:line_item_id) optimization in items is
  preserved. This arguably extends it: the one remaining association call per group now
  resolves from memory too.

Call sites updated

• Spree::Shipment#manifest
• spree/admin/orders/_shipment — this one builds the manifest from
  shipment.inventory_units.where(carton_id: nil), and already has order in scope.

Behaviour change to be aware of

A manifest built with order: now surfaces in-memory, unsaved changes to line items.
That is the intent, but it is observable: anything relying on the manifest's line item
being a clean read of the row will see the order's working copy instead.

Why not inverse_of?

InventoryUnit and LineItem already declare it on both sides, but inverse_of only
links objects loaded through one another. The shipment → inventory units → line item
path never passes through order.line_items.

Not addressed

first_unit.variant has the same shape and gets no equivalent treatment. Variants are
not mutated in memory during order recalculation the way adjustments are, so there is no
correctness bug there — only a preloading question, which callers can handle with
includes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.03%. Comparing base (e4e7df1) to head (61f9317).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6524   +/-   ##
=======================================
  Coverage   92.02%   92.03%           
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  Files        1035     1035           
  Lines       21141    21152   +11     
=======================================
+ Hits        19456    19467   +11     
  Misses       1685     1685           

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[tvdeyen]

Nice work!