fix(canvas): mock WorkspaceUsage in BudgetLimit.DetailsTab test

DetailsTab renders WorkspaceUsage alongside BudgetSection. The test suite
sets api.get to return [] (a valid empty peers list) but WorkspaceUsage
calls api.get for metrics and crashes on undefined input_tokens when the
mock returns an array instead of a WorkspaceMetrics object.

Add a stub vi.mock following the same pattern already used for BudgetSection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canvas/src/components/__tests__/BudgetLimit.DetailsTab.test.tsx

@@ -44,6 +44,15 @@ vi.mock("../tabs/BudgetSection", () => ({
   ),
 }));
 
+// Mock WorkspaceUsage — it has its own test suite (WorkspaceUsage.test.tsx).
+// Without this mock its internal api.get call races against the shared mock
+// and crashes when the return value is not a valid WorkspaceMetrics object.
+vi.mock("../WorkspaceUsage", () => ({
+  WorkspaceUsage: ({ workspaceId }: { workspaceId: string }) => (
+    <div data-testid="workspace-usage-stub" data-ws={workspaceId} />
+  ),
+}));
+
 import { api } from "@/lib/api";
 import { DetailsTab } from "../tabs/DetailsTab";