fix(http): 🐛 Fix RequestUtils.get_client() AttributeError

[Elity]

The get_client() method was returning cls.client which doesn't exist, causing AttributeError: type object 'RequestUtils' has no attribute 'client'

• 🔧 Change __client from class reference to None with type hint

• ✨ Implement lazy initialization pattern for HTTPClient instance

• 🐛 Fix incorrect attribute access (cls.client -> cls.__client)

【INFO】     2025-12-23 12:43:10 | 检测到 Alist2Strm 模块配置，正在添加至后台任务

Traceback (most recent call last):

  File "/app/main.py", line 39, in <module>

    Alist2Strm(**server).run, trigger=CronTrigger.from_crontab(cron)

    ^^^^^^^^^^^^^^^^^^^^

  File "/app/modules/alist2strm/alist2strm.py", line 66, in __init__

    self.client = AlistClient(url, username, password, token)

                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/app/utils/multiton.py", line 14, in __call__

    cls._instances[key] = super().__call__(*args, **kwargs)

                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/app/modules/alist/v3/client.py", line 37, in __init__

    self.__client = RequestUtils.get_client()

                    ^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/app/utils/http.py", line 351, in get_client

    return cls.client

           ^^^^^^^^^^

AttributeError: type object 'RequestUtils' has no attribute 'client'

Path objects do not support the context manager protocol. The 'with'
statement was incorrectly used with PosixPath objects, causing TypeError
on startup.

• 🔧 Remove invalid 'with' statement for CONFIG_DIR and LOG_DIR
• ✨ Directly use Path.exists() and Path.mkdir() methods

运行报错：
Traceback (most recent call last):
  File "/Users/user/code/AutoFilm/./app/main.py", line 10, in <module>
    from app.core import settings, logger
  File "/Users/user/code/AutoFilm/./app/core/__init__.py", line 1, in <module>
    from app.core.config import settings
  File "/Users/user/code/AutoFilm/./app/core/config.py", line 107, in <module>
    settings = SettingManager()
  File "/Users/user/code/AutoFilm/./app/core/config.py", line 26, in __init__
    self.__mkdir()
    ~~~~~~~~~~~~^^
  File "/Users/user/code/AutoFilm/./app/core/config.py", line 33, in __mkdir
    with self.CONFIG_DIR as dir_path:
         ^^^^^^^^^^^^^^^
TypeError: 'PosixPath' object does not support the context manager protocol