refactor Config() and _ENV_CONFIG

### Apache Iceberg version

None

### Please describe the bug 🐞

Follow up to #3006, i want to see if we can refactor Config() and _ENV_CONFIG
a couple of issues ive seen
* Config() `__init__` reads both env vars and local config files implicitly, this can be a potentially footgun.
* _ENV_CONFIG is loaded on import time... which calls Config() above and reads env vars and local config files.

#3006 is a fix for pytests. Ideally, I think we can be more explicit here

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

先追蹤 Config() 和 _ENV_CONFIG，接著閱讀後續 issue #3006，以了解現有的 pytest 修正以及目前的設定載入行為。當設定檔和環境變數不再由 Config() 或在匯入期間隱式讀取，且預期的明確行為已由測試涵蓋時，即表示完成。

由索引模型根據 Issue 內容生成。