test: fix insertContentTool mock to match actual implementation

- Update mock to return Promise<void> instead of Promise<{ success: boolean }>
- Aligns test mock with the actual tool implementation

Author: hannesrudolph

src/core/tools/__tests__/insertContentTool.spec.ts

@@ -96,6 +96,7 @@ describe("insertContentTool", () => {
 					finalContent: "final content",
 				}),
 				scrollToFirstDiff: vi.fn(),
+				updateDiagnosticSettings: vi.fn(),
 				pushToolWriteResult: vi.fn().mockImplementation(async function (
 					this: any,
 					task: any,