Add regression coverage for LLM fallback model option resolution

## Problem
The current test suite does not cover LLM fallback model option resolution.

## 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-model-options; Focus=LLM fallback model option resolution; PrTitle=test(llm): cover fallback model option resolution; TestFile=web/src/pages/studio/__tests__/llmModelOptions.test.ts; Mode=new; Append=/*
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import { describe, expect, it } from 'vitest';
import { FALLBACK_MODELS, fallbackModelOptions } from '../llmModelOptions';

describe('fallbackModelOptions', () => {
it('returns provider fallbacks as value/label options', () => {
expect(fallbackModelOptions('openai')).toEqual(
FALLBACK_MODELS.openai.map((item) => ({ value: item, label: item })),
);
});

it('uses the current model for unknown providers when one is supplied', () => {
expect(fallbackModelOptions('custom', 'gpt-5.6-sol')).toEqual([
{ value: 'gpt-5.6-sol', label: 'gpt-5.6-sol' },
]);
});

it('returns an empty list for an unknown provider without a current model', () => {
expect(fallbackModelOptions('custom')).toEqual([]);
});
});}.TestFile). No runtime behavior changes.

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

Start with web/src/pages/studio/__tests__/llmModelOptions.test.ts and the referenced fallbackModelOptions implementation in web/src/pages/studio/llmModelOptions. Run the targeted Vitest file first, then add regression coverage for provider fallbacks and unknown-provider behavior. Done means the targeted test passes and git diff --check is clean.

Written by the indexing model from the issue text.