storage: add regression test for upsert sources with keys that move partitions

[guswynn]

Closes #26965

cc @morsapaes

We only want to commit to the fact that if a keys partition only increases, then we do the right thing.

Motivation

• tests/verification

Checklist

• [x] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [ ] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [ ] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [ ] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [ ] This PR includes the following user-facing behavior changes: