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

首先定位使用 Field(description=...) 的 auth 相关 Pydantic 模型，然后查看 #1253 中的 V2 检查清单，以确定预期范围。将这些字段描述替换为标准 Python docstring，并验证所有目标 auth 模型不再使用此文档模式。