WA-NEW-007: Stabilize create_placed_order factory

[kitcommerce]

Summary

Harden the create_placed_order test factory:

• Re-apply selected shipping option to ensure Shipping#base_price is set
• Improve UnplacedOrderError message with checkout state diagnostics
• Fix overrides typo :update_at → :updated_at

Closes #624

Client impact

None. This is a test factory change only — no production code affected.

Verify

bundle exec ruby -Icore/test core/test/services/workarea/copy_order_test.rb

Note

This branch includes WA-NEW-006 and earlier commits (stacked). Those should land first or be reviewed together.

[kitcommerce]

Heads up: this PR was unstacked/rewritten as part of #637.

I extracted the earlier commits into separate PRs (#651, #646, #647, #648, #649) and force-pushed this branch so it now contains only the WA-NEW-007 commit (based directly on next).

If you had this branch checked out locally, you will need to re-fetch/reset.