fix: stabilize extension startup deadline validation

The total-deadline test relied on a timer firing at an exact millisecond boundary, so a shared runner could legally begin the second extension before observing expiration. Use the existing finite synchronous-work fixture to exceed the total budget deterministically while preserving the same production behavior assertion.

src/main/agent/deepseek-harness-extension-loader.test.ts

@@ -112,9 +112,10 @@ describe('DeepSeek Harness extension loader', () => {
 
   it('bounds the complete extension startup sequence', async () => {
     const ctx = new Context()
-    const importModule = vi.fn(
+    const importModule = vi.fn(async () => {
-      () => new Promise<never>(() => undefined)
+      blockEventLoop(25)
-    )
+      return {}
+    })
 
     const result = await loadControlledHarnessExtensions(
       ctx,