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>