Add regression coverage for agent engine store persistence and migration

## Problem
The current test suite does not cover agent engine store persistence and migration.

## Expected behavior
- A focused Vitest case locks the existing behavior.
- The targeted test file passes and fails if the covered behavior regresses.

## Scope
Only add regression coverage for $(@{Slug=engine-store; Focus=agent engine store persistence and migration; PrTitle=test(ai-engine): cover agent engine persistence and migration; TestFile=web/src/stores/__tests__/engineStore.test.ts; Mode=new; Append=/*
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import { afterEach, describe, expect, it, vi } from 'vitest';

const STORAGE_KEY = 'rocketmq-studio-agent-engine';

async function loadStore(persistedEngine?: string, persistedVersion = 1) {
vi.resetModules();
if (persistedEngine !== undefined) {
localStorage.setItem(
STORAGE_KEY,
JSON.stringify({ state: { engine: persistedEngine }, version: persistedVersion }),
);
}
return (await import('../engineStore')).useEngineStore;
}

describe('engineStore', () => {
afterEach(() => {
vi.resetModules();
localStorage.clear();
});

it('defaults to claude-code when no preference is persisted', async () => {
const store = await loadStore();
expect(store.getState().engine).toBe('claude-code');
});

it('restores a persisted supported engine', async () => {
const store = await loadStore('qoder');
expect(store.getState().engine).toBe('qoder');
});

it('updates the persisted engine through the action', async () => {
const store = await loadStore('claude-code');
store.getState().setEngine('http');
expect(store.getState().engine).toBe('http');
});

it('migrates stale persisted engines back to claude-code', async () => {
const store = await loadStore('stale-engine', 0);
expect(store.getState().engine).toBe('claude-code');
});
});}.TestFile). No runtime behavior changes.

## Acceptance criteria
- [ ] The targeted Vitest file passes.
- [ ] git diff --check is clean.

Start with web/src/stores/__tests__/engineStore.test.ts and inspect the imported ../engineStore implementation to understand its persistence and migration behavior. Run the targeted Vitest file, covering the stated default, restore, update, and migration cases. Done means the targeted test passes and git diff --check is clean.

Written by the indexing model from the issue text.