fix: 添加接口数据格式的验证，避免list返回影响整个循环

[tdh62]

修复特别情况下返回list格式数据影响整个循环的问题

2025-09-20 14:49:31,000 | INFO | 🔍 Processing file: https://api.github.com/repos/kiang/dpp.org.tw/contents/tag?keyword=台北市.html
2025-09-20 14:49:33,127 | ERROR | --------------------------------------------------
| 💥 Unexpected error: 'list' object has no attribute 'get'
└--------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/app/app/hajimi_king.py", line 345, in main
    valid_count, rate_limited_count = process_item(item)
                                      ^^^^^^^^^^^^^^^^^^
  File "/app/app/hajimi_king.py", line 145, in process_item
    content = github_utils.get_file_content(item)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/utils/github_client.py", line 186, in get_file_content
    encoding = file_metadata.get("encoding")
               ^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'get'