fix: use Pi user auth and handle startup failures

backend/test/routes-sessions.test.ts

@@ -164,6 +164,9 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
       sessionList: async () => [{ id: "s1" }],
     } as any,
     getSettings: () => ({ workspace: "w", provider: "zai", model: "glm-5.2", thinking: "high" }),
+    listModels: async () => [
+      { provider: "zai", id: "glm-5.2", name: "GLM 5.2", reasoning: true },
+    ],
     spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
   });
   const created = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
@@ -259,6 +262,9 @@ test("POST /sessions configura Pi con il thinking globale selezionato", async ()
       sessionNew: async () => ({ id: "s-thinking" }),
     } as any,
     getSettings: () => ({ workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "high" }) as any,
+    listModels: async () => [
+      { provider: "zai", id: "glm-5.2", name: "GLM 5.2", reasoning: true },
+    ],
   });
 
   await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
@@ -1539,6 +1545,69 @@ test("POST /sessions proceeds when ollamaEnsure succeeds", async () => {
   expect(ensureWs).toBe("psd");
 });
 
+test("POST /sessions rejects an unavailable saved model before persisting a session", async () => {
+  let created = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      sessionNew: async () => { created += 1; return { id: "must-not-exist" }; },
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({
+      workspace: "psd",
+      provider: "deepseek",
+      model: "deepseek-v4-pro",
+      thinking: "medium",
+    }) as any,
+    listModels: async () => [
+      { provider: "zai", id: "glm-5.2", name: "GLM 5.2", reasoning: true },
+    ],
+  });
+
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+
+  expect(res.statusCode).toBe(503);
+  expect(res.json()).toEqual({
+    error: "Selected model is unavailable. Check Pi authentication and model settings, then try again.",
+    code: "model_unavailable",
+  });
+  expect(created).toBe(0);
+});
+
+test("POST /sessions marks a persisted session failed when runtime construction throws", async () => {
+  let failed = 0;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    mgr: {
+      createFor: () => { throw new Error("provider bootstrap unavailable"); },
+    } as any,
+    thtRunner: {
+      sessionNew: async () => ({ id: "s-runtime-failure" }),
+      failSession: async (id: string, workspace: string) => {
+        expect(id).toBe("s-runtime-failure");
+        expect(workspace).toBe("psd");
+        failed += 1;
+      },
+    } as any,
+    readiness: { ensure: async () => ({ ok: true }) } as any,
+    getSettings: () => ({
+      workspace: "psd",
+      provider: "deepseek",
+      model: "deepseek-v4-pro",
+      thinking: "medium",
+    }) as any,
+    listModels: async () => [
+      { provider: "deepseek", id: "deepseek-v4-pro", name: "DeepSeek V4 Pro", reasoning: true },
+    ],
+  });
+
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+
+  expect(res.statusCode).toBe(503);
+  expect(res.json()).toEqual({
+    error: "Session startup failed. Check configuration and connectivity, then Resume the session.",
+  });
+  expect(failed).toBe(1);
+});
+
 test("POST /sessions/:id/resume returns 409 for a read-only session without calling ollamaEnsure", async () => {
   let ensureCalled = false;
   const app = mutApp({