Add stricter checks for the Get BNPL URL Request being complete

This CL adds stricter checks for the
GetBnplPaymentInstrumentForFetchingUrlRequest being complete, as
incomplete requests can cause issues later during the client flow. If
the client has received an incomplete request, the flow should be
aborted and it should be treated as a failure.

Bug: 356443046
Change-Id: I2f1a23715db45d66fdce61714aa02c87e569cf58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6237115
Reviewed-by: Darwin Yang <darwinyang@chromium.org>
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Commit-Queue: Slobodan Pejic <slobodan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1417531}