What happens if all the necessary testing doesn't occur in a Sprint?

The Product Owner does not have to be a single person but may be a committee or a shared responsibility between multiple individuals.

During a backlog refinement meeting, a senior team member raises a concern about an epic sizing that requires the use of a new interface for a vendor product. The product owner acknowledges this as a risk.
What should the product owner do now?

In the first part of the Sprint Planning meeting, what is NOT accomplished?

Which Agile estimation technique is based upon relative sizing?