OAuth handler doesn't support redirect URLs with params

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

The oauth redirect handler uses the following logic (https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/auth/oauth2.py#L356-L357) to attach parameters to the redirect URL supplied by the server:

        authorization_url = f"{auth_endpoint}?{urlencode(auth_params)}"
        await self.context.redirect_handler(authorization_url)

Unfortunately, this breaks if the auth_endpoint supplied by the server already has URL parameters. For example, at https://api.salesforce.com/.well-known/oauth-authorization-server/platform/mcp/v1/sandbox/platform/sobject-all you can see that the salesforce mcp server has an authorization endpoint of https://test.salesforce.com/services/oauth2/authorize?prompt=select_account.

The end result is that the client tries to visit a URL that loooks like https://test.salesforce.com/services/oauth2/authorize?prompt=select_account?response_type=code&..., which is incorrect.

Example Code

Python & MCP Python SDK

python3 3.13.2
mcp 1.27.2

從 src/mcp/client/auth/oauth2.py 的第 356-357 行開始，這裡會在呼叫 redirect_handler 之前組合授權 URL。檢查伺服器提供的端點和授權參數是如何拼接的。當已經包含查詢參數的端點能夠產生同時包含現有參數和新加入參數的有效 URL 時，即視為完成。