Client Credentials In the token Handler

Is your feature request related to a problem? Please describe.
The MCP Authorization Spec 2.1.1 OAuth Grant Types mentions that both Authorization Code and Client Credentials should be handled.

Describe the solution you'd like
In /server/auth/handlers/token.py, we only currently support "authorization-code". It would be fantastic if we could support "client-credentials" as well.

https://github.com/modelcontextprotocol/python-sdk/blob/544176770b53e6a0ae8c413d3b6c5116421f67df/src/mcp/server/auth/handlers/token.py#L26-L47

Describe alternatives you've considered
I mean it's just more convenient, so I don't have to do the whole PKCE exchange thing if it's not necessary.

Additional context

Link to MCP spec: https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#2-1-1-oauth-grant-types

先閱讀 /server/auth/handlers/token.py，尤其是現有的 authorization-code 處理，並將其與 issue 中連結的 MCP OAuth Grant Types 規範進行比較。確認預期的 client-credentials 行為，並找出 repository 中相關的測試或測試入口點。當 token handler 支援 client-credentials 以及 authorization-code，且不要求 PKCE 交換時，即表示完成。