REST API: Adding + removing coupons via API doesn't recalculate totals correctly

[ryelle]

This PR is proof of an issue, rather than a fix in itself. When coupons are added or removed from an existing order via the API, the item totals are not updated. I've added two failing tests in this PR, one for adding & one for removing a coupon.

You can also replicate the issue manually, by sending requests to POST orders/:orderId

Add a coupon:

{
    "coupon_lines": [
        {
            "code": <coupon-code>
        }
    ]
}

Remove a coupon:

{
    "coupon_lines": [
        {
            "id": <couponLineId>,
            "code": null
        }
    ]
}

In both cases, the line items are not updated (the discount is not applied to line items, or it's not removed from line items), which means the order total is not updated again when calculate_totals is run. It looks like a fix might be to run WC_Order::recalculate_coupons if coupon_lines changes, but that's a protected function in the abstract class. An alternate approach could be to use the apply_coupon/remove_coupon functions directly for coupon_lines.

Feel free to take over this PR to fix this, or give me a pointer & I'll fix it. We'll probably end up porting whatever fix we end up with into https://github.com/woocommerce/wc-api-dev, so letting @justinshreve & @timmyc know this exists :)

[claudiulodro]

I think it needs to use WC_Order::apply_coupon/WC_Order::remove_coupon instead of manually setting/removing coupon order items. I'll open a PR to this PR.

[codecov]

Codecov Report

Merging #18236 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master   #18236      +/-   ##
============================================
+ Coverage     37.62%   37.64%   +0.02%     
  Complexity    12547    12547              
============================================
  Files           344      344              
  Lines         46812    46812              
============================================
+ Hits          17612    17621       +9     
+ Misses        29200    29191       -9

Impacted Files	Coverage Δ	Complexity Δ
includes/abstracts/abstract-wc-order.php	70.86% <0%> (+0.16%)	283% <0%> (ø)	⬇️
includes/class-wc-tax.php	80% <0%> (+0.26%)	132% <0%> (ø)	⬇️
includes/api/class-wc-rest-orders-controller.php	88.71% <0%> (+1.11%)	128% <0%> (ø)	⬇️

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[mikejolley]

@claudiulodro Merging these updated tests and logged a new issue for wc-api-dev here: woocommerce/wc-api-dev#78