mcp: remove support for batching on streamable connections

[wucm667]

Fixes #911

Batching was already deprecated and rejected for protocol versions >= 2025-06-18.
Remove it entirely to simplify the streamable transport code.

Replaces readBatch calls with single message decoding, removes batch-related
checks and comments, and deletes batch-specific tests. No behavioral change for
single-message requests.

[maciej-kisiel]

Thanks for the contribution. We're still analyzing our backwards compatibility stance on this and when exactly we would like to remove it. I'll update you when this is resolved.

[piyushbag]

Conceptually this looks right for #911: dropping readBatch and the batch-only branches simplifies servePOST without changing the single-message path.

GitHub shows the branch as conflicting with current main (recent streamable changes landed since May). Could you rebase onto latest main and push? Happy to take another look once CI is green.

After rebase, one thing I'd sanity-check: any remaining callers that still assumed batch decode on the stateless POST probe path.

[wucm667]

Rebased onto latest main and pushed. All CI checks are green.

Regarding the stateless POST probe path — ephemeralConnectOpts now uses jsonrpc2.DecodeMessage directly instead of readBatch, so there's no batch decode assumption left.

@maciej-kisiel mentioned they're still evaluating the backwards compatibility stance. Let me know if you have any other concerns.