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

首先阅读 src/mcp/client/auth.py 第 250 行附近的代码，目前这里会选择 PRM 文档中的第一个授权服务器。确定开发者应如何指定其首选服务器，并验证当文档中存在多个服务器时是否会遵循该选择。