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

Comece rastreando o processamento de requisições Streamable HTTP e o exemplo check_accept_headers na issue, com foco em como os parâmetros Accept são analisados. Verifique o comportamento com a requisição fornecida application/json;q=0 e text/event-stream;q=1.0 e considere casos adicionais de fatores de qualidade; considera-se concluído quando tipos de mídia explicitamente não aceitáveis não são tratados como aceitos segundo a semântica da RFC 7231.