v8.6.0+ can fail when locking signing data element

[olemartinorg]

Description of the bug

It seems that before v8.6.0, signing data elements were not locked when leaving the signing task, but in v8.6.0 (specifically after #956) these data elements are locked when leaving the task. However, if the user does not have write permissions (only sign), this will fail. Our own test-app, ttd/signing-test can thus not be upgraded to v8.6.0 and is stuck on v8.5.6 for now.

Relates to Altinn/altinn-studio#16640, as upgrading all test-apps to run with project references (basically using app-lib-dotnet main) is a requirement for running Cypress tests in our monorepo.

Steps To Reproduce

1. Upgrade ttd/signing-test to v8.6.0 or newer
2. Run the Cypress tests or try the app manually
3. Observe that process/next fails

Additional Information

Long thread with a few dead-ends here: https://digdir.slack.com/archives/C08ELTRPFV3/p1761300192245339

[bjorntore]

A couple of suggestions:

1. Do something similar as in PutInstanceAndEvents in Storage: Allow locking if you have "sign", instead of "write", if the task type is signing.
2. Change it so that storage just locks the data element at the same time the signature is saved to the db, and make sure further attempts to lock it again from the app are ignored? Might not work.

[olemartinorg]

Until this gets fixed, I've made a short-circuit in the e2e tests in Altinn/altinn-studio#16936, so they still pass even though this bug is present. That clutter should obviously be removed when this problem is resolved.