fix: #3612555 Hide the 'Change seats' link on a settled order: re-seating only applies to a pending basket

Fixes #3612555. Follows up #3612196 / #3612364.

Problem

On a settled order (confirmed, completed or cancelled) the order summary still showed a "Change seats" link on each seated event. Re-seating only moves the visitor's held seats through their basket (SeatSelectionController acts on the cart's held lines and the current visitor's cart), so the link is a dead end once the order has left the basket, and on an order detail page it would act on the current visitor's own cart rather than the order shown.

Fix

hook_yoyaku_summary_event_alter received only the slot, so seating could not tell the order's state. TransactionSummary::build() now passes the transaction as a third, backward-compatible context argument, and yoyaku_seating offers the "Change seats" link only while the order is pending.

Tests

New kernel test SummaryEventLinkTest: the link shows for a pending basket and is gone once the order is completed. The existing 2-argument test double still works, confirming the extra context arg is backward compatible.