fix(test): hermetically exercise auth workflows

backend/test/enabled-models.test.ts

@@ -1,4 +1,4 @@
-import { expect, test } from "vitest";
+import { expect, test, vi } from "vitest";
 import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -35,6 +35,20 @@ test("loads exact global enabledModels in configured order", () => {
   } finally { rmSync(f.root, { recursive: true, force: true }); }
 });
 
+test("uses the configured Pi agent directory when no explicit directory is passed", () => {
+  const f = fixture({ enabledModels: ["zai/glm-5.2"] });
+  vi.stubEnv("PI_CODING_AGENT_DIR", f.agentDir);
+  try {
+    expect(loadPiEnabledModels({ harnessDir: f.harnessDir })).toMatchObject({
+      ids: ["zai/glm-5.2"],
+      warnings: [],
+    });
+  } finally {
+    vi.unstubAllEnvs();
+    rmSync(f.root, { recursive: true, force: true });
+  }
+});
+
 test("project enabledModels overrides global enabledModels", () => {
   const f = fixture(
     { enabledModels: ["zai/glm-5.2", "zai/glm-5v-turbo"] },