fix: make session resume atomic across restarts

frontend/src/shell/AppShell.session-mgmt.test.tsx

@@ -134,6 +134,60 @@ test("an already-active same-session Resume preserves its EventSource and store"
   expect(useSessionStore.getState().activityLog).toEqual(before);
 });
 
+test("a false then true pair of concurrent Resume completions preserves the cold stream", async () => {
+  let resumeCalls = 0;
+  let releaseCold!: () => void;
+  let releaseAlreadyActive!: () => void;
+  let markColdStarted!: () => void;
+  let markAlreadyActiveStarted!: () => void;
+  const coldStarted = new Promise<void>((resolve) => { markColdStarted = resolve; });
+  const alreadyActiveStarted = new Promise<void>((resolve) => { markAlreadyActiveStarted = resolve; });
+  const coldReleased = new Promise<void>((resolve) => { releaseCold = resolve; });
+  const alreadyActiveReleased = new Promise<void>((resolve) => { releaseAlreadyActive = resolve; });
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/resume", async () => {
+      resumeCalls += 1;
+      if (resumeCalls === 1) {
+        markColdStarted();
+        await coldReleased;
+        return resumeResult("s1", false);
+      }
+      markAlreadyActiveStarted();
+      await alreadyActiveReleased;
+      return resumeResult("s1", true);
+    }),
+    http.get("http://localhost:8787/sessions/:id", () =>
+      HttpResponse.json({ id: "s1", status: "open", phase: 1 })),
+  );
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  const resumeButton = await screen.findByRole("button", { name: /resume/i });
+
+  await userEvent.click(resumeButton);
+  await coldStarted;
+  await userEvent.click(resumeButton);
+  await alreadyActiveStarted;
+  expect(resumeCalls).toBe(2);
+
+  releaseCold();
+  await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
+  const stream = FakeEventSource.instances[0];
+  act(() => stream.emitNamed(
+    "ui_request",
+    { type: "ui_request", ui_request: { id: "cold-gate", widget: "select" } },
+    "1",
+  ));
+  const before = useSessionStore.getState().activityLog.map((entry) => ({ ...entry }));
+
+  releaseAlreadyActive();
+  await waitFor(() => expect(screen.queryByText("Domanda originale")).not.toBeInTheDocument());
+
+  expect(FakeEventSource.instances).toHaveLength(1);
+  expect(stream.closed).toBe(false);
+  expect(useSessionStore.getState().pendingWidget?.id).toBe("cold-gate");
+  expect(useSessionStore.getState().activityLog).toEqual(before);
+});
+
 test("cold same-session Resume keeps the old stream until success then receives post-clear events once", async () => {
   server.use(
     http.post("http://localhost:8787/sessions/:id/resume", () =>
@@ -146,7 +200,7 @@ test("cold same-session Resume keeps the old stream until success then receives
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
   await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
   const first = FakeEventSource.instances[0];
-  act(() => first.emitNamed("info", { type: "info", text: "Before cold Resume" }, "7"));
+  act(() => first.emitNamed("info", { type: "info", text: "Before cold Resume" }, "900"));
 
   let releaseResume!: () => void;
   let markStarted!: () => void;
@@ -168,7 +222,7 @@ test("cold same-session Resume keeps the old stream until success then receives
   expect(useSessionStore.getState().activityLog).toContainEqual({
     kind: "status", phase: "F1", text: "Before cold Resume", level: "info",
   });
-  act(() => first.emitNamed("text_delta", { type: "text_delta", text: "Still attached" }, "8"));
+  act(() => first.emitNamed("text_delta", { type: "text_delta", text: "Still attached" }, "901"));
   expect(useSessionStore.getState().transcript).toEqual([
     { role: "assistant", text: "Still attached" },
   ]);
@@ -177,7 +231,7 @@ test("cold same-session Resume keeps the old stream until success then receives
   await waitFor(() => expect(FakeEventSource.instances).toHaveLength(2));
   expect(first.closed).toBe(true);
   const replacement = FakeEventSource.instances[1];
-  expect(replacement.url).toContain("/sessions/s1/events?lastEventId=8");
+  expect(replacement.url).toBe("http://localhost:8787/sessions/s1/events");
   expect(useSessionStore.getState().transcript).toEqual([]);
   expect(useSessionStore.getState().activityLog).toEqual([
     { kind: "lifecycle", phase: null, text: "Resuming session" },
@@ -186,7 +240,7 @@ test("cold same-session Resume keeps the old stream until success then receives
   act(() => replacement.emitNamed(
     "text_delta",
     { type: "text_delta", text: "Post-resume delivery" },
-    "9",
+    "1",
   ));
   expect(useSessionStore.getState().transcript).toEqual([
     { role: "assistant", text: "Post-resume delivery" },
@@ -199,8 +253,8 @@ test("cold same-session Resume keeps the old stream until success then receives
     ui_request: { id: "post-resume-gate", widget: "select", title: "Review once" },
   };
   act(() => {
-    replacement.emitNamed("ui_request", gate, "10");
-    replacement.emitNamed("ui_request", gate, "11");
+    replacement.emitNamed("ui_request", gate, "2");
+    replacement.emitNamed("ui_request", gate, "3");
   });
   expect(useSessionStore.getState().pendingWidget?.id).toBe("post-resume-gate");
   expect(useSessionStore.getState().activityLog.filter((entry) => entry.kind === "gate"))