Reject server messages that are not JSON-RPC 2.0 in the client transports

[koic]

Motivation and Context

Issue #589 reports that MCP::Client#call_tool returns normally when the server's response omits jsonrpc or sets it to "1.0". The bundled transports never read that member of an incoming message, while the server side of this SDK answers -32600 to such a request and the TypeScript, Python, Go, and C# SDKs validate every incoming message against a schema whose jsonrpc is the literal "2.0".

MCP::Client::Stdio and MCP::Client::HTTP now check it. A response that is not a JSON object carrying "jsonrpc": "2.0" fails the request with RequestHandlerError instead of being returned: over stdio the frame answering the awaited id, over HTTP the JSON body or the first response-shaped SSE event, on a resumed stream as well. Over HTTP that is what the Python client does with the answer to a request, and the TypeScript client with a JSON body. Over stdio both of them report the message as an error and keep reading; this transport reads synchronously and its read_timeout defaults to none, so reading on would wait forever, and the request fails at once instead. A server-to-client request that is not JSON-RPC 2.0 is ignored: a ping is not answered and a handler is not called. Frames for other ids keep being skipped, the body answering a notification is not checked, and under mode: :auto a rejected server/discover answer falls back to the legacy handshake like any other probe failure. An empty or null JSON body of a 200 is still handed over as nil, as before.

The check lives in the transports, not in MCP::Client#request, so a custom transport keeps handing over whatever Hash it builds.

The HTTP client tests stubbed most JSON bodies as { result: ... } without the member, which the transport now rejects,
so those 54 stubs and three whole-response assertions carry jsonrpc: "2.0".

Closes #589.

How Has This Been Tested?

test/mcp/client/stdio_test.rb and test/mcp/client/http_test.rb. Against the library before this change, 20 of the 22 new tests fail; the other two pin behavior that stays the same.

Breaking Changes

A server whose responses lack "jsonrpc": "2.0" no longer works with the bundled transports, and neither does a test that stubs such a response for them: the member has to be sent, and there is no opt-out. Servers built with an MCP SDK send it already, and MCP::Client over a custom transport or a test double is unaffected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context