test(app-api): Fix api tests fixture race - #1875

Description

Altinn.App.Api.Tests has been failing intermittently in CI. The two failure modes seen in recent runs are both races between test classes that xUnit runs in parallel inside one test process, on the shared on-disk instance fixtures under Data/Instances. They are not related to which target framework runs the tests: each test process already gets its own copy of the fixtures, and the failures happened inside that private copy.

• Shared instance between two test classes. OptionsControllerTests (11 tests) and DataController_LayoutEvaluatorTests both used instance 500600/cff1cb24-5bc1-4888-8e06-c634753c5144 in tdd/contributer-restriction. TestData.PrepareInstance deletes and recreates the instance's data files, so while an OptionsControllerTests test prepared the instance, the layout evaluator test's PUT to the data element failed with FileNotFoundException (run, run). The fixture is copied to a new instance GUID (and data element GUID) that only DataController_LayoutEvaluatorTests uses.
• Recursive enumeration of the instances tree in the mock. InstanceClientMockSi.GetInstancePath(partyId, guid) located the instance file with Directory.GetFiles(..., SearchOption.AllDirectories) over the whole instances directory. That descends into the data folders of every other instance, including one that a parallel test is deleting through TestData.DeleteInstanceAndData. On Windows a directory in pending-delete state is reported as access denied, so the enumeration threw and e.g. InstancesController_PostNewInstanceTests.PostNewInstanceWithContent_ValidFilename_Succeeds got a 500 (run). The lookup now probes the fixed org/app/partyId/guid.json layout, walking only the org and app directories, which no test deletes. GetInstances is changed the same way: it walks the remaining directory levels and lists only the *.json files at the instance level, so it reads the same files as before without enumerating into the instance folders.

The old party filter in GetInstancePath was a substring match on the path, so party 1337 also matched 501337. The new lookup compares the directory name exactly.

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Altinn.App.Api.Tests was run locally on both target frameworks after each commit: 467 tests pass on net10.0 and 466 on net8.0. Three further net10.0 runs only failed in AddRuntimeConfigFiles_Production_ReloadsWhenKubernetesDataSymlinkChanges, which times out waiting for a file-watcher event in the sandbox used for testing, fails the same way on unmodified main there, and is skipped on Windows in CI.

The Windows race can only be reproduced on Windows, so that part is verified by reasoning about the code and by the test run, not by reproducing the original failure.

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)