fix(chat): handle raw structured session NOT_FOUND errors from Tauri

Elargit isNotFound() pour reconnaitre aussi la forme brute
'not found: structured session <uuid>' que Tauri peut retourner,
en plus de la forme typée {code: 'NOT_FOUND'}.

Ajoute deux tests couvrant:
- fallback sur reattach avec erreur brute
- retry prompt sur send avec erreur brute

QA: tests passés sur cette correction.

frontend/src/features/agents/CustomAgentChatView.test.tsx

@@ -152,6 +152,65 @@ describe("CustomAgentChatView", () => {
     expect(screen.queryByText(/structured session gone/)).toBeNull();
   });
 
+  it("falls back when Tauri surfaces structured session NOT_FOUND as a raw string", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => ({
+        sessionId: "chat-session-2",
+        assignedConversationId: "conversation-2",
+      })),
+      reattachAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce(
+          "not found: structured session 69b57638-0ac7-4a79-8252-e0436a3265f6",
+        )
+        .mockImplementationOnce(async (sessionId: string) => ({
+          sessionId,
+          scrollback: [],
+        })),
+      sendAgentChat: vi.fn(() => new Promise<void>(() => {})),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const onSessionId = vi.fn();
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="69b57638-0ac7-4a79-8252-e0436a3265f6"
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() => expect(agent.launchAgentChat).toHaveBeenCalledTimes(1));
+    expect(agent.reattachAgentChat).toHaveBeenNthCalledWith(
+      1,
+      "69b57638-0ac7-4a79-8252-e0436a3265f6",
+      expect.any(Function),
+    );
+    expect(agent.reattachAgentChat).toHaveBeenNthCalledWith(
+      2,
+      "chat-session-2",
+      expect.any(Function),
+    );
+    expect(onSessionId).toHaveBeenCalledWith(null);
+    expect(onSessionId).toHaveBeenCalledWith("chat-session-2");
+    expect(screen.queryByText(/not found: structured session/)).toBeNull();
+  });
+
   it("does not reopen when the parent echoes a launched session id", async () => {
     const agent = {
       launchAgentChat: vi.fn(async () => ({
@@ -286,6 +345,74 @@ describe("CustomAgentChatView", () => {
     expect(screen.queryByText(/not found: structured session/)).toBeNull();
   });
 
+  it("recovers and retries the prompt when send reports structured session NOT_FOUND as a raw string", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => ({
+        sessionId: "chat-session-2",
+        assignedConversationId: "conversation-2",
+      })),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce("not found: structured session stale-session")
+        .mockImplementationOnce(async (_sessionId: string, _prompt: string, onChunk) => {
+          onChunk({ kind: "final", content: "Recovered reply" });
+        }),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+    const onSessionId = vi.fn();
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="stale-session"
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "stale-session",
+        expect.any(Function),
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText(/message CLI custom/), {
+      target: { value: "please recover" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+
+    await waitFor(() => expect(agent.sendAgentChat).toHaveBeenCalledTimes(2));
+    expect(agent.sendAgentChat).toHaveBeenNthCalledWith(
+      2,
+      "chat-session-2",
+      "please recover",
+      expect.any(Function),
+    );
+    expect(onSessionId).toHaveBeenCalledWith(null);
+    expect(onSessionId).toHaveBeenCalledWith("chat-session-2");
+    expect(await screen.findByText("Recovered reply")).toBeTruthy();
+    expect(screen.queryByText(/not found: structured session/)).toBeNull();
+  });
+
   it("treats NOT_FOUND during cancel as an already-gone session", async () => {
     const agent = {
       launchAgentChat: vi.fn(),

frontend/src/features/agents/CustomAgentChatView.tsx

@@ -54,7 +54,16 @@ function instrumentationError(e: unknown): Record<string, unknown> {
 }
 
 function isNotFound(e: unknown): boolean {
-  return Boolean(e && typeof e === "object" && (e as GatewayError).code === "NOT_FOUND");
+  if (e && typeof e === "object" && (e as GatewayError).code === "NOT_FOUND") {
+    return true;
+  }
+  const message =
+    typeof e === "string"
+      ? e
+      : e && typeof e === "object" && "message" in e
+        ? String((e as GatewayError).message)
+        : "";
+  return /^not found: structured session\b/i.test(message);
 }
 
 function unknownChunkLabel(chunk: unknown): string {