[fix] perfect structure decode for complex string

[Aur0ra-m]

Summary

Fixes #7402

By implementing a stack-based processing logic, we can elegantly resolve the issue of nested fenced blocks while avoiding the pitfalls of aggressive stripping that compromise complex structural outputs.

Model output:
```json
    {
        "name": "test",
        "value": "### Notes
    Use the following snippet:
    ```python
    def greet():
        print('hello')
    ```",
        "description": "Markdown includes python code block"
    }
    ```

Type of change

• Bug fix
• New feature
• Breaking change
• Improvement
• Model update
• Other: