Support selection of authorization servers from PRM document

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

SDK should offer a way for developers to specify which of the authorization servers to use from the PRM document.

https://github.com/modelcontextprotocol/python-sdk/blob/07ae8c0d4e58bc0527a6898709288686ef6506dd/src/mcp/client/auth.py#L250

Right now, the first server is selected.

Example Code

Python & MCP Python SDK

Latest

先閱讀 src/mcp/client/auth.py 約第 250 行的位置，目前這裡會選取 PRM 文件中的第一個授權伺服器。確定開發者應如何指定其偏好的伺服器，並驗證當文件中存在多個伺服器時是否會遵循該選擇。