CVE-2026-52869: MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session. Anyone who learned or guessed a session ID could send JSON-RPC messages on that session, regardless of which bearer token the request carried.