Add regression coverage for LLM test failure result fallbacks

## Problem
The current test suite does not cover LLM test failure result fallbacks.

## 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=llm-failure-result; Focus=LLM test failure result fallbacks; PrTitle=test(llm): cover failure result fallbacks; TestFile=web/src/pages/studio/__tests__/llmFailureResult.test.ts; Mode=new; Append=/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

import { describe, expect, it } from 'vitest';
import { buildLlmFailureResult } from '../llmFailureResult';

describe('buildLlmFailureResult', () => {
it('falls back to the caller message when the API result is missing', () => {
expect(buildLlmFailureResult(null, '连接失败')).toEqual({
success: false,
msg: '连接失败',
code: undefined,
hint: undefined,
});
});

it('prefers the API error message and preserves optional diagnostics', () => {
expect(
buildLlmFailureResult(
{ status: 0, errMsg: 'invalid key', code: 'INVALID_KEY', hint: '检查密钥' },
'连接失败',
),
).toEqual({
success: false,
msg: 'invalid key',
code: 'INVALID_KEY',
hint: '检查密钥',
});
});
});}.TestFile). No runtime behavior changes.

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

Create web/src/pages/studio/__tests__/llmFailureResult.test.ts and use the existing buildLlmFailureResult entry point from ../llmFailureResult. Run the targeted Vitest file first, then verify the fallback and diagnostic cases pass and git diff --check is clean.

Written by the indexing model from the issue text.