fix(backend): restart idle Pi sessions on resume

backend/test/routes-sessions.test.ts

@@ -133,6 +133,56 @@ test("POST /sessions/:id/resume usa il thinking globale se manca nel manifest",
   expect(configured.thinking).toBe("low");
 });
 
+test.each(["running", "waiting"])(
+  "POST resume preserves a %s runtime",
+  async (state) => {
+    let tornDown = false;
+    const existing = { bridge: { turnState: () => state } } as any;
+    const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+      mgr: {
+        get: () => existing,
+        teardown: () => { tornDown = true; },
+      } as any,
+      thtRunner: {} as any,
+      getSettings: () => ({ workspace: "psd" }) as any,
+    });
+    const response = await app.inject({ method: "POST", url: "/sessions/s1/resume" });
+    expect(response.json()).toEqual({ id: "s1", alreadyActive: true });
+    expect(tornDown).toBe(false);
+  },
+);
+
+test.each(["idle", "failed"])(
+  "POST resume replaces a %s runtime and starts the persisted session",
+  async (state) => {
+    const order: string[] = [];
+    const oldRuntime = { bridge: { turnState: () => state } } as any;
+    const newRuntime = { bridge: { onClientEvent: () => {}, emitClientEvent: () => {} } } as any;
+    const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+      mgr: {
+        get: () => oldRuntime,
+        teardown: (id: string) => order.push(`teardown:${id}`),
+        createFor: () => { order.push("create"); return newRuntime; },
+        configure: async () => {},
+        start: () => order.push("start"),
+      } as any,
+      thtRunner: {
+        sessionShow: async () => ({
+          status: "open", archived: false,
+          provider: "local-qwen", model: "qwen3.6-35b-a3b", thinking: "low",
+        }),
+        reopenSession: async () => order.push("reopen"),
+      } as any,
+      readiness: { ensure: async () => ({ ok: true }) } as any,
+      getSettings: () => ({ workspace: "local", thinking: "medium" }) as any,
+    });
+    const response = await app.inject({ method: "POST", url: "/sessions/s1/resume" });
+    await new Promise((resolve) => setImmediate(resolve));
+    expect(response.json()).toEqual({ id: "s1" });
+    expect(order).toEqual(["teardown:s1", "reopen", "create", "start"]);
+  },
+);
+
 test("POST /sessions/:id/response inoltra al bridge (no error)", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {