fix: strip markdown JSON code blocks in OpenAI adapter responseFormat

[best]

Closes #54

Problem

When using responseFormat: { type: 'json' }, many LLMs (especially OpenAI-compatible endpoints like MiniMax) return JSON wrapped in markdown code fences:

```json
{"name": "Alice", "age": 30}

This causes `JSON.parse()` failures in downstream consumers.

## Solution

Added `stripJsonCodeBlock()` utility to `openai-adapter.ts` that detects and removes markdown code fences from responses when `responseFormat.type === 'json'`.

### Changes
- `packages/ai/src/adapters/openai-adapter.ts` — Add `stripJsonCodeBlock()`, call it in `doGenerate()` after `stripThinkTags()`
- `packages/ai/src/index.ts` — Export `stripJsonCodeBlock` from `@openlinkos/ai`
- `packages/ai/__tests__/strip-json-code-blocks.test.ts` — 8 unit tests (new file)

### Verification
- All 1033 tests pass (including 8 new stripJsonCodeBlock tests)
- No existing tests broken
- Only affects responses when `responseFormat.type === 'json'`

[github-actions]

🤖 AI Code Review

Automated review for PR #55 — informational only, does not block merge.

Checklist

	Item	Notes
✅	Type Safety	No issues detected
✅	Error Handling	No issues detected
✅	Test Coverage	1 test file(s) updated
✅	API Consistency	1 new export(s) added
✅	Breaking Changes	No breaking changes detected

Stats

• 📁 Files changed: 3 (3 TypeScript)
• ➕ Lines added: 79
• 🧪 Test files updated: 1

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All checks passed — looks good! 🎉

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[best]

🔍 代码审查 — PR #55

关联 Issue: #54 — responseFormat JSON 返回 markdown 包裹导致解析失败

方案： stripJsonCodeBlock() 自动剥离 markdown code fence

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✅ 审查通过

• 方案精准：仅在 responseFormat.type === "json" 时触发
• 执行顺序正确：stripThinkTags() 之后调用
• 正则健壮：lazy match + 首尾锚定，仅整段包裹才剥离
• 测试覆盖全：8 个用例，含边界和负面测试
• 改动最小化：3 文件，核心逻辑约 20 行

注意事项（不阻塞）：

• 当前仅覆盖非流式 doGenerate() 路径，流式场景如需支持可后续扩展

CI 全绿： Build ✅ | Test ✅ | AI Review ✅ | Release check ✅

已执行 squash merge。