[TOC] includes headings inside list items (e.g. `- ### Case1`) in the table of contents

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

or Chinese version: https://hackmd.io/c/tutorials-tw/

### What's the problem

#### Current behaviour

When using `[TOC]`, headings written inside list items are also included in the table of contents.

Reproducible content:

```md
[TOC]

### Section
- ### Case1
- ### Case2
- ### Case3
````

Actual rendered TOC result:

```md
- Section
- Case1
- Case2
- Case3
```

Provide steps to reproduce:

1. Go to [https://hackmd.io](https://hackmd.io)

2. Create a new note

3. Paste the content below:

```md
[TOC]

### Section
- ### Case1
- ### Case2
- ### Case3
```

4. Switch to preview / rendered view

5. Observe that `Case1`, `Case2`, and `Case3` are also included in TOC

> Here is an example: https://hackmd.io/mhkr3m9WR4Kse_ePoV_c6w
> (This never happened before.)

#### Expected behaviour

I would expect only the main section heading to appear in the table of contents:

```md
- Section
```

Headings inside list items such as `- ### Case1` should ideally not be included in TOC, or this behavior should be clearly documented if it is intended.

#### Environment

I use HackMD on:

**Desktop**

* OS: Windows / Ubuntu 24.04 / Android 14
* Browser: Chrome 147.0.7727.56 , FireFox 143.0（64 位元）

**Mobile**

* Device: Xiaomi Pro 13
* OS: Android 14
* Browser: Chrome 146.0.7880.177

#### Additional context

This may be intended Markdown parsing behavior, but from a TOC usability perspective it is surprising and can pollute the generated table of contents.
If this is expected behavior, documentation clarification would be helpful.