fix: Blobs._getOnePending() can Error on multiple results

[alxndrsn]

Closes #1373

What has been done to verify that this works as intended?

Run tests 400 times:

• https://github.com/alxndrsn/odk-central-backend/actions/runs/13131375879/attempts/1
• https://github.com/alxndrsn/odk-central-backend/actions/runs/13131375879/attempts/2
• https://github.com/alxndrsn/odk-central-backend/actions/runs/13131375879/attempts/3
• https://github.com/alxndrsn/odk-central-backend/actions/runs/13131375879/attempts/4
• https://github.com/alxndrsn/odk-central-backend/actions/runs/13131375879/attempts/5

Why is this the best possible solution? Were any other approaches considered?

It seems simple... however, there are currently no tests which will immediately detect a regression, or similar issues in future. Any suggestions?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Should fix the bug for users.

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

No.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced