Validation on attachment upload may block subsequent navigation

[olemartinorg]

#3641 adds useAttachmentUpdateTagsMutation(), which in turn updates initial validations on success (if the backend version is new enough). When working on #3741 I upgraded ttd/expression-validation-test to the latest backend, but found the cypress tests for it to be failing and getting stuck on NavigationButtons. It seems the infinite loading that occurs in that failing test is caused by it comparing initial validations with an expected value and never succeeding. My theory is that useAttachmentUpdateTagsMutation() should not update initial validation at all, because that endpoint is not running full (but rather incremental) validation.

Tasks here:

• Investigate to verify the endpoint is running incremental validation
• Fix the implementation to store returned validations properly
• Add cypress tests for this functionality to verify it still works as expected
• Test the fix against this app, and notify in the thread: https://digdir-samarbeid.slack.com/archives/C02EJ9HKQA3/p1758616258486659