Replace Field(description=...) with proper docstrings in auth models

From the V2 checklist in #1253 (under "Refactor the auth way of using it"):

Drop the use of Field(description=...), and use proper docstrings. As far as I know, we are not creating json schemas from it, so it's useless.

The auth-related Pydantic models currently use Field(description=...) to document fields. Since we are not generating JSON schemas from these models, the descriptions are not surfaced anywhere useful. They should be replaced with standard Python docstrings that are more discoverable via IDEs and help().

This is part of the broader auth refactor planned for V2.

Ref: Kludex's V2 checklist in #1253

先找出使用 Field(description=...) 的 auth 相關 Pydantic 模型，接著檢視 #1253 中的 V2 檢查清單，以確認預定範圍。將這些欄位描述替換為標準 Python docstring，並確認所有目標 auth 模型不再使用此文件模式。