Fix Streamable HTTP Accept header quality-factor handling

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

Streamable HTTP currently ignores q=0 values in the Accept header, so requests that explicitly mark a media type as unacceptable can still be accepted. This violates RFC 7231 semantics for content negotiation and can cause clients and servers to disagree about which response format is actually allowed.

Example Code

from starlette.requests import Request

def check_accept_headers(request: Request) -> tuple[bool, bool]:
    accept_header = request.headers.get("accept", "")
    accept_types = [media_type.strip().split(";")[0].strip().lower() for media_type in accept_header.split(",")]

    has_wildcard = "*/*" in accept_types
    has_json = has_wildcard or any(t in ("application/json", "application/*") for t in accept_types)
    has_sse = has_wildcard or any(t in ("text/event-stream", "text/*") for t in accept_types)

    return has_json, has_sse

# Example: client explicitly rejects JSON
request = Request({"type": "http", "method": "GET", "headers": [(b"accept", b"application/json;q=0, text/event-stream;q=1.0")]})

print(check_accept_headers(request))
# -> (True, True)

Python & MCP Python SDK

Python 3.12.10
MCP Python SDK: 1.28.1

先追蹤 Streamable HTTP 請求處理流程以及 issue 中的 check_accept_headers 範例，重點關注 Accept 參數的解析方式。使用提供的 application/json;q=0 和 text/event-stream;q=1.0 請求驗證行為，並考慮其他 quality factor 情況；完成的標準是，在 RFC 7231 語義下，明確不可接受的媒體類型不會被當作已接受。