bug: markitdown fails with 403 Forbidden when converting Zhihu article URL

When attempting to convert a Zhihu article using the CLI tool, markitdown throws an unhandled exception due to a 403 Forbidden response from the target URL.

Reproduction Steps:

markitdown https://zhuanlan.zhihu.com/p/11654788270 > test.md

Error Traceback:

Traceback (most recent call last):
  File "/Users/xiongxinwei/Library/Caches/pypoetry/virtualenvs/telepace-server-WT4oou3h-py3.12/bin/markitdown", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File ".../markitdown/__main__.py", line 197, in main
    result = markitdown.convert(
             ^^^^^^^^^^^^^^^^^^^
  File ".../markitdown/_markitdown.py", line 271, in convert
    return self.convert_uri(source, stream_info=stream_info, **_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../markitdown/_markitdown.py", line 443, in convert_uri
    response.raise_for_status()
  File ".../site-packages/requests/models.py", line 1024, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://zhuanlan.zhihu.com/p/11654788270

Expected Behavior:

The tool should either:

• Successfully fetch and convert the article if access is allowed, or
• Gracefully handle 403 responses with a clear error message indicating access is denied.

Environment:

• OS: macOS
• Python: 3.12
• Tool version: latest from repo
• Installed via: Poetry

Possible Cause:

Zhihu may be blocking automated requests. It might be necessary to:

• Add custom headers (e.g., a user-agent string) to mimic a browser
• Handle HTTP errors more gracefully

Start in markitdown/_markitdown.py at convert_uri, where the traceback shows response.raise_for_status(), and review the CLI entry point in markitdown/main.py. Reproduce the Zhihu URL with the documented command, then verify that a 403 is handled with a clear access-denied error or that conversion succeeds when access is allowed.