feat(FN-3573): harden plugin setup sync and test-isolation handling

- Add plugin setup API coverage for migration and sync edge cases
- Expand plugin route tests and implementation safeguards for setup state handling
- Update legacy API glue to align plugin setup responses with route behavior
- Keep test-isolation runtime ignore handling compatible with live app activity

Fusion-Task-Id: FN-3573

scripts/check-test-isolation.mjs

@@ -186,6 +186,7 @@ function checkAgainstBaseline() {
   const baselineByDir = new Map((baseline.protectedFusion ?? []).map((entry) => [entry.dir, entry]));
   const unstableProtectedDirs = new Set(baseline.unstableProtectedDirs ?? []);
   const currentProtected = snapshotProtectedFusion();
+
   const candidateViolations = [];
   for (const current of currentProtected) {
     if (unstableProtectedDirs.has(current.dir)) continue;