Streamable HTTP: Prevent batching with JSONResponse=true and protocol >= 2025-06-18

[findleyr]

In #21, we restricted JSON batching support when the protocol version is >= 2025-06-18 (when it was removed). However, we missed the ad-hoc batching done in 'JSON mode', when the server author has indicated responses should be application/json:

go-sdk/mcp/streamable.go

Line 733 in 920e5a3

// TODO: add tests for batch responses, or disallow them entirely.

We must remove this behavior on recent protocol versions. Necessarily, this will either requiring preventing servers from sending non-response messages in the context of handling a request, or force them to fall back on text/event-stream.

[findleyr]

Looking at the code more closely, this is actually done (though not obviously so). Nevertheless, it will be simplified in an upcoming CL.