Module exec leads to crash

The following scene fails on startup:

```py
from dataclasses import dataclass
from manimlib import *

@dataclass
class Coordinates2D:
x: float
y: float

class MyScene(Scene):
def construct(self):
print("Hello, world!")

```

Start it via

```bash
manimgl "./path/to/bug-test-file.py" MyScene -e 13
```

and you get the following traceback (Python version: `3.12.8`):

```
Traceback (most recent call last):
File "/home/dominic/dev/splience-manim-videos/venv/bin/manimgl", line 8, in
sys.exit(main())
^^^^^^
File "/home/dominic/dev/manim-fork/manimlib/__main__.py", line 26, in main
reload_manager.run()
File "/home/dominic/dev/manim-fork/manimlib/reload_manager.py", line 47, in run
self.retrieve_scenes_and_run()
File "/home/dominic/dev/manim-fork/manimlib/reload_manager.py", line 81, in retrieve_scenes_and_run
scenes = manimlib.extract_scene.main(scene_config, run_config)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/dominic/dev/manim-fork/manimlib/extract_scene.py", line 173, in main
module = get_scene_module(
^^^^^^^^^^^^^^^^^
File "/home/dominic/dev/manim-fork/manimlib/extract_scene.py", line 168, in get_scene_module
insert_embed_line_to_module(module, embed_line)
File "/home/dominic/dev/manim-fork/manimlib/extract_scene.py", line 162, in insert_embed_line_to_module
exec(code_object, module.__dict__)
File ".home.dominic.dev.splience-manim-videos.manimbug", line 5, in
File "/usr/lib/python3.12/dataclasses.py", line 1275, in dataclass
return wrap(cls)
^^^^^^^^^
File "/usr/lib/python3.12/dataclasses.py", line 1265, in wrap
return _process_class(cls, init, repr, eq, order, unsafe_hash,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/dataclasses.py", line 983, in _process_class
and _is_type(type, cls, dataclasses, dataclasses.KW_ONLY,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/dataclasses.py", line 749, in _is_type
ns = sys.modules.get(cls.__module__).__dict__
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute '__dict__'. Did you mean: '__dir__'?
```

The latest commit where it works correctly is df1e0674. Then, in the next commit 1a14a6bd it doesn't work anymore. Thus, #2262 has introduced this bug. It's not by chance that this PR is called "Refactor scene creation".

該錯誤會在 manimgl 執行包含 dataclass 的 scene 檔案時發生。traceback 指向 manimlib/extract_scene.py，具體是 insert_embed_line_to_module 中的 exec 呼叫。將 commit 1a14a6bd（來自 PR #2262）的變更與可用的 commit df1e0674 比較。執行提供的測試腳本以重現崩潰，然後檢查場景擷取期間模組 namespace 的處理方式。

由索引模型根據 Issue 內容生成。