401 in Streamable HTTP should be handled gracefully

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

Currently, streamable_http will call response.raise_for_status() when handling POST requests. (ref).

This means that a 401 will bubble up and cause the entire program to crash.

While a 401 on server initialize could warrant an exception, we have a use case where a MCP server will return 401 for specific unauthorized operations, and allow others.

This should be handled gracefully, similar to how _handle_unexpected_content_type works, returning a JSONRPC message outlining an unauthorized exception was thrown.

This would let an agent or other client handle it instead of crashing the program.

Python & MCP Python SDK

Python 3.12 and MCP SDK v1.13.1

从 src/mcp/client/streamable_http.py 中 response.raise_for_status() 附近的 POST 处理开始，然后将其与 _handle_unexpected_content_type 进行比较。跟踪 HTTP 错误如何变成客户端结果；完成的标准是：未授权操作返回的 401 会产生一条 JSON-RPC unauthorized 错误消息，客户端可以处理该消息而不会崩溃。