fix(#310): merge reordered one-time items by normalizing empty subscription option

[chrismshea]

Fixes #310

Problem

Reordering the same one-time product twice from My Account → My Orders creates two separate quote line items instead of merging. Same duplicate-row behavior occurs when adding a subscription-enabled product as a one-time purchase from PLP/PDP repeatedly.

Root cause

Plugin\Quote\Item::compareOptions decided mergeability by the subscription_option value in info_buyRequest. A freshly reordered item's raw buyRequest has no subscription_option key, while Plugin\Sales\Model\Reorder\ReorderPlugin stamps option=onetime_purchase onto items in afterExecute — i.e. after Magento\Sales\Model\Reorder\Reorder::execute has already run the merge comparison and persisted the item.

On the second reorder, the persisted item carries option=onetime_purchase while the incoming item is still unstamped (null), so compareOptions returns "not equal" and Magento creates a new line item. This is a regression introduced alongside #191 (ReorderPlugin, 1.7.0); before it, reordered items had no subscription_option at all and merged as equal-empty.

Fix

Normalize a missing/empty subscription option to onetime_purchase before comparing. An item with no subscription option is semantically a one-time purchase. This makes merge behavior independent of the afterExecute stamping order and also covers the controller reorder path (AbstractController\Reorder) that ReorderPlugin doesn't hook, plus repeated PLP/PDP one-time adds.

Genuine subscription-vs-one-time and interval mismatches are unaffected.

Tests

• All 6 existing ItemTest data-provider cases still pass under the new logic.
• Added a regression case: stamped onetime_purchase item vs unstamped item (no subscription_option key) → must merge.

Verification

• vendor/bin/phpunit Test/Unit/Plugin/Quote/ItemTest.php green
• Manual: place a one-time order → Reorder twice from My Orders → one merged line item (qty incremented), not two
• Manual: add a subscription-enabled product as one-time from PDP/PLP repeatedly → merges
• Regression: subscription add + interval selection still behaves as before