feat(rendezvous): libellé explicite des notifications de fin de background task (#91)

Le toast de fin de tâche de fond liée à un rendez-vous inter-agent passe
de « Main -> DevBackend completed » à « Appel Main -> DevBackend terminé »
(idem en échec / annulé), pour nommer explicitement l'appel plutôt qu'un
état brut. Fallback générique conservé quand requester/target sont absents.

Validations obtenues avant commit : tests frontend annoncés verts pour #91.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/projects/ProjectsView.ls7.test.tsx

@@ -266,7 +266,7 @@ describe("ProjectsView — LS7 conversation viewer integration", () => {
     });
 
     const toast = await screen.findByRole("button", {
-      name: /Main -> DevBackend completed/i,
+      name: /Appel Main -> DevBackend terminé/i,
     });
     expect(within(toast).getByText("Task task-ren")).toBeTruthy();
 
@@ -277,10 +277,57 @@ describe("ProjectsView — LS7 conversation viewer integration", () => {
     ).toBeTruthy();
     expect(await screen.findByText("contenu du fil ouvert")).toBeTruthy();
     expect(
-      screen.queryByRole("button", { name: /Main -> DevBackend completed/i }),
+      screen.queryByRole("button", { name: /Appel Main -> DevBackend terminé/i }),
     ).toBeNull();
   });
 
+  it("labels rendezvous failure and cancellation toasts with explicit call attribution", async () => {
+    const project = new MockProjectGateway();
+    const created = await project.createProject("alpha", "/p/a");
+    const agent = new MockAgentGateway();
+    const system = new MockSystemGateway();
+    const requester = await agent.createAgent(created.id, {
+      name: "Main",
+      profileId: "codex",
+    });
+    const target = await agent.createAgent(created.id, {
+      name: "DevBackend",
+      profileId: "codex",
+    });
+    renderView(project, { agent, system });
+
+    await openProjectAndWorkTab("/p/a");
+    system.emit({
+      type: "backgroundTaskChanged",
+      projectId: created.id,
+      agentId: target.id,
+      taskId: "task-rendezvous-failed",
+      state: "failed",
+      requesterAgentId: requester.id,
+      targetAgentId: target.id,
+    });
+    system.emit({
+      type: "backgroundTaskChanged",
+      projectId: created.id,
+      agentId: target.id,
+      taskId: "task-rendezvous-cancelled",
+      state: "cancelled",
+      requesterAgentId: requester.id,
+      targetAgentId: target.id,
+    });
+
+    expect(
+      await screen.findByRole("button", {
+        name: /Appel Main -> DevBackend en échec/i,
+      }),
+    ).toBeTruthy();
+    expect(
+      await screen.findByRole("button", {
+        name: /Appel Main -> DevBackend annulé/i,
+      }),
+    ).toBeTruthy();
+  });
+
   it("keeps the generic background-task toast when rendezvous agent ids are absent", async () => {
     const project = new MockProjectGateway();
     const created = await project.createProject("alpha", "/p/a");