fix(auth): close diagnostic filesystem races

backend/test/auth-request-snapshot.test.ts

@@ -7,6 +7,7 @@ import { buildApp, type AppWithAuthSessionStore } from "../src/app.js";
 import { loadAuthenticationConfig } from "../src/auth/config.js";
 import type { AuthenticationConfigProvider, LoadedAuthConfig } from "../src/auth/types.js";
 import { loadConfig } from "../src/config.js";
+import { prepareAuthStateRoot } from "./auth-test-fixtures.js";
 
 const password = "correct horse battery staple";
 const passwordHash = "$argon2id$v=19$m=65536,t=3,p=1$AAECAwQFBgcICQoLDA0ODw$DRo8ZSPI8G5OCvnFFapbVEjP69aDjy1Sw9i2743cPC4";
@@ -78,9 +79,11 @@ async function createFixture(first: "A" | "B", later: "A" | "B" | "oidc") {
       return calls === 1 ? snapshots[first] : snapshots[later === "oidc" ? "B" : later];
     },
   };
+  const authStateRoot = join(directory, "auth-state");
+  prepareAuthStateRoot(authStateRoot);
   const config = loadConfig({
     THT_AUTH_CONFIG_FILE: authA,
-    THT_AUTH_STATE_ROOT: join(directory, "auth-state"),
+    THT_AUTH_STATE_ROOT: authStateRoot,
     THT_HARNESS_DIR: "/tmp/h",
   });
   config.authentication = provider;