[iOS] Handle bad provider in Payments Bottom Sheet gracefully

In the unlikely event that the triggering field's type changes
(possibly due to a mismatch between local and server field
predictions) between the moment the Payments Bottom Sheet opens
and the moment a suggestion is used, this CL adds a safety exit
to handle this corner case gracefully. A new entry was added to
"exit reasons" to see how often this case happens.

Bug: 1513412
Change-Id: I050eabaeeef1992449adaaa13858308e80697487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5146348
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1240235}