OAuthClientProvider._handle_token_response expect json

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

https://github.com/modelcontextprotocol/python-sdk/blob/673423da0d6511127608df3d75360e47e99cb1f1/src/mcp/client/auth.py#L437
OAuthClientProvider._handle_token_response expects JSON, but doesn't specify this in the headers. If the server responds in a different format, an exception will be raised.

For example, https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#2-users-are-redirected-back-to-your-site-by-github
access_token endpoint by default returns form-data and requires the Accept: application/json header to return json.

Example Code

Python & MCP Python SDK

python 3.11.9
mcp=1.16.0

從 src/mcp/client/auth.py 中大約第 437 行的 _handle_token_response 參照處開始，然後閱讀連結的 GitHub OAuth 文件，了解 access-token 回應格式。檢查 provider 如何處理伺服器回應，並透過確認文件所述的 form-data 回應不再引發例外，同時仍支援 JSON 回應來驗證完成情況。