fix: add listMissions to MissionStore mock to prevent flaky test

The MissionLoop calls this.missionStore.listMissions() during startup
recovery, but the mock TaskStore's getMissionStore() return value didn't
include this method. When the runtime startup sequence raced ahead, it
would hit "listMissions is not a function" — making the test flaky.

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

packages/engine/src/runtimes/__tests__/in-process-runtime.test.ts

@@ -57,6 +57,7 @@ vi.mock("@fusion/core", async () => {
       self.getTask = mockTaskStoreGetTask;
       self.getSettings = vi.fn().mockImplementation(async () => structuredClone(mockTaskStoreSettings));
       self.getMissionStore = vi.fn().mockReturnValue({
+        listMissions: vi.fn().mockReturnValue([]),
         getMissionWithHierarchy: vi.fn().mockReturnValue(null),
         findNextPendingSlice: vi.fn().mockReturnValue(null),
         activateSlice: vi.fn(),