[2025-03-26 spec] Implement JSON-RPC batching

[mikekistler]

For compliance with the 2025-03-25 Model Context Protocol spec:

• Add support for JSON-RPC batching (PR #228)

[halter73]

As of #330, this is implemented for the server-side Streamable HTTP transport, but not for stdio or the Streamable HTTP client yet.

[halter73]

Closing this as not planned since it has been removed from the specification. See modelcontextprotocol/modelcontextprotocol#416 for more context.