feat(chat): valide le routing cellKind=chat côté frontend et corrige UX

- Ajoute cellKind dans AgentChatHandle et vérifie que launchAgentChat reçoit cellKind:chat
- Rejette avec erreur STRUCTURED_ROUTED_TO_PTY si le backend route vers PTY
- Ajoute cellKind:chat dans MockAgentGateway.launchAgentChat
- Corrige l'overflow du layout CustomAgentChatView (bounded shell avec scroll + composer fixe)
- Dédouble les prompts utilisateur quand le stream les echo
- Ajoute les tests launchAgentChat returns handle only when confirms chat et launchAgentChat rejects PTY-routed launch
- Ajoute les tests does not duplicate user prompt when echoed et keeps chat shell bounded

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -152,6 +152,112 @@ describe("CustomAgentChatView", () => {
     expect(screen.queryByText(/structured session gone/)).toBeNull();
   });
 
+  it("does not duplicate the user prompt when the stream echoes it back", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi.fn(async (_sessionId: string, _prompt: string, onChunk) => {
+        onChunk({ kind: "userPrompt", text: "hello agent" });
+        onChunk({ kind: "final", content: "done" });
+      }),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    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="chat-session-1"
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "chat-session-1",
+        expect.any(Function),
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText(/message CLI custom/), {
+      target: { value: "hello agent" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Envoyer" }));
+
+    await screen.findByText("done");
+    expect(screen.getAllByText("hello agent")).toHaveLength(1);
+  });
+
+  it("keeps the chat shell bounded with a scrollable message area and fixed composer", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(() => new Promise<never>(() => {})),
+      reattachAgentChat: vi.fn(),
+      sendAgentChat: vi.fn(() => new Promise<void>(() => {})),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker with a very long name that must not collide with controls"
+          profile={{
+            ...profile,
+            name: "Structured profile with a very long display name",
+          }}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId={null}
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    const shell = screen.getByTestId("custom-agent-chat-view");
+    const toolbar = await screen.findByRole("toolbar", {
+      name: "custom agent chat actions",
+    });
+    const scroll = screen.getByTestId("custom-agent-chat-scroll");
+    const composer = screen.getByTestId("custom-agent-chat-composer");
+    const cancel = await screen.findByRole("button", { name: "Cancel" });
+
+    expect(shell.className).toContain("h-full");
+    expect(shell.className).toContain("min-h-0");
+    expect(shell.className).toContain("overflow-hidden");
+    expect(toolbar.className).toContain("overflow-hidden");
+    expect(cancel.className).toContain("shrink-0");
+    expect(scroll.className).toContain("flex-1");
+    expect(scroll.className).toContain("basis-0");
+    expect(scroll.className).toContain("overflow-y-auto");
+    expect(composer.className).toContain("shrink-0");
+  });
+
   it("falls back when Tauri surfaces structured session NOT_FOUND as a raw string", async () => {
     const agent = {
       launchAgentChat: vi.fn(async () => ({
@@ -476,6 +582,7 @@ describe("CustomAgentChatView", () => {
     const agent = {
       launchAgentChat: vi.fn(async () => ({
         sessionId: "new-session",
+        cellKind: "chat",
         assignedConversationId: "conversation-2",
       })),
       reattachAgentChat: vi.fn().mockRejectedValueOnce({
@@ -514,6 +621,49 @@ describe("CustomAgentChatView", () => {
     );
   });
 
+  it("does not publish a session id when custom launch is refused as PTY-routed", async () => {
+    const agent = {
+      launchAgentChat: vi.fn().mockRejectedValueOnce({
+        code: "STRUCTURED_ROUTED_TO_PTY",
+        message:
+          "custom CLI launch for agent agent-1 in project project-1 returned cellKind=pty; expected chat. sessionId=pty-session-1; nodeId=node-1",
+      }),
+      reattachAgentChat: vi.fn(),
+      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={null}
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    const alert = await screen.findByRole("alert");
+    expect(alert.textContent).toContain("cellKind=pty");
+    expect(agent.launchAgentChat).toHaveBeenCalledTimes(1);
+    expect(agent.reattachAgentChat).not.toHaveBeenCalled();
+    expect(onSessionId).not.toHaveBeenCalled();
+  });
+
   it("recovers when post-launch reattach reports NOT_FOUND", async () => {
     const agent = {
       launchAgentChat: vi