fix(chat): fallback vers un nouveau lancement quand la session structurée reattach est morte (#147)

reattachAgentChat pouvait échouer avec NOT_FOUND (session structurée fermée
ou jamais vivante) et laisser fuiter l'erreur brute jusqu'à l'utilisateur
("not found: structured session ... not found") au lieu de relancer une
session fraîche. Le fallback ne s'applique qu'au code NOT_FOUND ; toute
autre erreur de reattach continue de remonter telle quelle.

QA vert 2026-08-05 : npx vitest run CustomAgentChatView.test.tsx,
npx vitest run agents.test.tsx CustomAgentChatView.test.tsx, npm run typecheck.

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

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

@@ -94,4 +94,100 @@ describe("CustomAgentChatView", () => {
       expect.any(Function),
     );
   });
+
+  it("falls back to a fresh launch when reattach reports NOT_FOUND", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(async () => ({
+        sessionId: "chat-session-2",
+        assignedConversationId: "conversation-2",
+      })),
+      reattachAgentChat: vi
+        .fn()
+        .mockRejectedValueOnce({ code: "NOT_FOUND", message: "structured session gone" })
+        .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="stale-session"
+          conversationId="conversation-1"
+          onSessionId={onSessionId}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.launchAgentChat).toHaveBeenCalledWith("project-1", "agent-1", {
+        cwd: "/repo",
+        rows: 24,
+        cols: 80,
+        conversationId: "conversation-1",
+        nodeId: "node-1",
+      }),
+    );
+    await waitFor(() =>
+      expect(onSessionId).toHaveBeenCalledWith("chat-session-2"),
+    );
+    expect(screen.queryByText(/structured session gone/)).toBeNull();
+  });
+
+  it("surfaces non-NOT_FOUND reattach errors instead of launching a new session", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn().mockRejectedValueOnce({
+        code: "PROCESS",
+        message: "structured session failed to attach",
+      }),
+      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"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="existing-session"
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    expect((await screen.findByRole("alert")).textContent).toContain(
+      "structured session failed to attach",
+    );
+    expect(agent.launchAgentChat).not.toHaveBeenCalled();
+  });
 });

frontend/src/features/agents/CustomAgentChatView.tsx

@@ -154,11 +154,18 @@ export function CustomAgentChatView({
       setError(null);
       try {
         if (sessionId) {
-          const reattached = await agent.reattachAgentChat!(sessionId, receive);
-          if (cancelled) return;
-          setCurrentSession(reattached.sessionId);
-          setTurns(reattached.scrollback.reduce(foldChunk, [] as ChatTurn[]));
-          return;
+          try {
+            const reattached = await agent.reattachAgentChat!(sessionId, receive);
+            if (cancelled) return;
+            setCurrentSession(reattached.sessionId);
+            setTurns(reattached.scrollback.reduce(foldChunk, [] as ChatTurn[]));
+            return;
+          } catch (e) {
+            // The backend contract for `reattach_agent_chat` (NOT_FOUND) is that
+            // the session is gone (closed/never live) and the caller falls back
+            // to a fresh launch — any other error still surfaces to the user.
+            if ((e as GatewayError)?.code !== "NOT_FOUND") throw e;
+          }
         }
         const launched = await agent.launchAgentChat!(projectId, agentId, {
           cwd,