Product variant pages generated using ProductVariation->toUrl() don't have the right cache contexts

So in my testing the issue seems to be that the render array's cache metadata does not include the 'v' query string key, and so the fields relating to the current product variation from the context are cached regardless of that value. This patch adds the cache context to the build array.

Not sure if test coverage in Commerce exists/needs to change to cover this?

That is the correct approach. I thought there was an open issue for this already. Good patch. We can write tests for it ensure cache busting.

Let's add a test and wrap this up.

+++ b/modules/product/src/ProductViewBuilder.php
@@ -61,6 +61,7 @@ class ProductViewBuilder extends EntityViewBuilder {
     $product_type = $product_type_storage->load($entity->bundle());
     if ($product_type->shouldInjectVariationFields() && $entity->getDefaultVariation()) {
+      $build['#cache']['contexts'][]  = 'url.query_args:v';

Actually, I'm pretty sure we can define this on the product class itself.

This doesn't add tests, but it puts the cache context on the product entity so it is always available and bubbles up.

Committed #7 since it's release day, renaming the issue so that it receives the test.

Reverting the title, having a test for this one-liner no longer feels like good use of our time.