Ensure JSONRPCError messages are sent for streamable http

[mhart]

Prior to this, any JSONRPCError message sent when enableJsonResponse is true would be ignored (and response.end() never called)

Motivation and Context

There's a bug when enableJsonResponse is true and an error is sent, say, by the protocol. The current logic will just ignore any message that isn't a JSONRPCResponse – and JSONRPCError is one of these.

How Has This Been Tested?

I tested this by sending a message that would parse, but fail at the protocol layer due to no handler being found.

{"jsonrpc":"2.0","id":"0","method":"a"}

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines

Additional context