test(stats): add childrenByParentId to task-page handler catalog mocks

The root-only task page reads childrenByParentId for childTasks;
the handler/routing specs' catalog stubs predated that index.

Author: Zoo (VP)

src/core/webview/__tests__/usageStatsMessageHandler.spec.ts

@@ -1775,6 +1775,7 @@ describe("usageStatsMessageHandler", () => {
 				catalogRevision: 7,
 				getPage: vi.fn(() => ({ tasks: ["history-task"], cursor: "next", totalEstimate: 1 })),
 				getDescendantTaskIds: vi.fn(() => []),
+				childrenByParentId: new Map(),
 				byId: new Map([["history-task", { id: "history-task", task: "History task", ts: 321 }]]),
 				ancestorsByTaskId: new Map(),
 			}
@@ -1808,6 +1809,7 @@ describe("usageStatsMessageHandler", () => {
 				catalogRevision: 7,
 				getPage: vi.fn(() => ({ tasks: [], cursor: undefined, totalEstimate: 0 })),
 				getDescendantTaskIds: vi.fn(() => []),
+				childrenByParentId: new Map(),
 				byId: new Map(),
 				ancestorsByTaskId: new Map(),
 			}

src/core/webview/__tests__/usageStatsMessageRouting.spec.ts

@@ -426,6 +426,7 @@ describe("usageStatsMessageRouting", () => {
 				catalogRevision: 2,
 				getPage: vi.fn(() => ({ tasks: ["task-001"], cursor: undefined, totalEstimate: 1 })),
 				getDescendantTaskIds: vi.fn(() => []),
+				childrenByParentId: new Map(),
 				byId: new Map([["task-001", { id: "task-001", task: "History task", ts: 100 }]]),
 				ancestorsByTaskId: new Map(),
 			}