SmartReplaceObjects: ImportAsset(ForceSynchronousImport) no longer reliable after PR#2 test ordering shift

[Scaler0222]

Background

SelectionDrivenSkillTests.SmartReplaceObjects_ReplacesSelectedObjectsWithPrefab flakes on Unity 6000.5.0b8 + URP 17.5. Failure mode: Expected: "success" / But was: "error" at SelectionDrivenSkillTests.cs:137 — the smart_replace_objects skill returns an error envelope because AssetDatabase.LoadAssetAtPath<GameObject>(prefabPath) (inside the skill body) returns null even after the test's prefab SaveAsPrefabAsset call.

PR#1 (#3) added a one-line guard at SelectionDrivenSkillTests.cs:129:

PrefabUtility.SaveAsPrefabAsset(sourcePrefabGo, prefabPath);
Object.DestroyImmediate(sourcePrefabGo);
AssetDatabase.ImportAsset(prefabPath, ImportAssetOptions.ForceSynchronousImport);

That fix was sufficient at PR#1 verification time (test passed cleanly).

PR#2 (#5) added 3 new tests in sibling fixtures (SkillsModeManagerTests.CurrentMode_ExplicitAutoPref_…, …CurrentMode_MigrationMatrix_AllThreeBranchesHonorIntent, SkillValidationTests.Execute_UnknownParamErrorEnvelope_PreservesNestedDetailsPath_ForLegacyClients). The new tests don't touch any Smart code path, but they're enough to shift NUnit's test-discovery order so that something running before SmartReplaceObjects_… leaves the AssetDatabase in a state where ForceSynchronousImport no longer reliably stabilizes the import.

Observed in 3 of 4 post-PR#2 verification runs.

Root cause hypothesis

Unity 6 import pipeline is genuinely async — the worker is a separate process, and AssetDatabase.LoadAssetAtPath<GameObject> reads from the imported-asset cache (not raw disk). On 6000.5.0b8, ImportAssetOptions.ForceSynchronousImport blocks until the import message is processed, but if the worker is mid-flight on a prior test's prefab (or scene save) the synchronization isn't atomic across the Save → Import → Load chain.

The Smart skill body (SmartSkills.cs:516-537) does not retry on null load — first miss → "Prefab not found" envelope → test fails.

Recommended fix

Test-side hardening — wrap save-and-load in a small retry loop in SelectionDrivenSkillTests.cs:

var prefabPath = Path.Combine(TempRoot, "Replacement.prefab").Replace('\', '/');
PrefabUtility.SaveAsPrefabAsset(sourcePrefabGo, prefabPath);
Object.DestroyImmediate(sourcePrefabGo);

// Belt-and-suspenders for Unity 6.x async import: flush, refresh, then
// wait for the prefab to actually appear in the imported-asset cache.
AssetDatabase.SaveAssets();
AssetDatabase.Refresh();
AssetDatabase.ImportAsset(prefabPath, ImportAssetOptions.ForceSynchronousImport);
for (var attempt = 0; attempt < 5 && AssetDatabase.LoadAssetAtPath<GameObject>(prefabPath) == null; attempt++)
{
    System.Threading.Thread.Sleep(50);
    AssetDatabase.ImportAsset(prefabPath, ImportAssetOptions.ForceSynchronousImport);
}
Assert.That(AssetDatabase.LoadAssetAtPath<GameObject>(prefabPath), Is.Not.Null,
    "Prefab must be visible in the imported-asset cache before the skill is called");

Alternative: harden the skill itself in SmartSkills.SmartReplaceObjects with a single retry-on-null. That's a bigger contract change for the production skill and not preferred — the test environment is the brittle thing.

Acceptance criteria

• SmartReplaceObjects_ReplacesSelectedObjectsWithPrefab passes on 50/50 consecutive EditMode runs on Unity 6000.5.0b8 + URP 17.5.
• No change to Editor/Skills/SmartSkills.cs.
• No change to other passing SelectionDrivenSkillTests cases.

Context

Surfaced during PR#2 (#5) verification of issue #1. PR#1 ImportAsset fix is still in place — needed but not sufficient under the new NUnit ordering. Tracking separately so PR#3 (ShaderGraph) can ship.

Refs: #1
Related: #3 (PR#1, added the initial ImportAsset guard), #5 (PR#2, added the test additions that shifted ordering)