Handle non-object JSON-RPC messages without raising

[koic]

Motivation and Context

A JSON-RPC message is an object, but two stdio-facing paths assumed that and raised on a non-object value parsed from an incoming frame:

• JsonRpcHandler.handle maps each element of a batch array through process_request, which reads request[:id] assuming a Hash. A non-object batch element (for example [[]] or [5]) makes [][:id] raise TypeError. handle_json rescued only JSON::ParserError, so the error escaped; over stdio it escaped the StdioTransport#open read loop. The HTTP transport already rejects array bodies before dispatch, so this surfaced only on stdio.
• MCP::Client::Stdio#read_response parses each frame from the server and calls parsed.key?("id"), which raises NoMethodError when the server emits a non-object frame. Only JSON::ParserError was rescued, so one malformed line from the server escaped.

Both are robustness gaps: a malformed frame should be reported or skipped, not raise an unhandled exception out of the transport.

• process_request now returns an INVALID_REQUEST error response for a non-object request before touching request[:id], so a malformed batch element becomes a normal per-element error.

• handle_json gains a last-resort StandardError rescue that returns an INTERNAL_ERROR response, so an unexpected handling error still produces a JSON-RPC reply.

• read_response skips a non-object frame the same way it skips a frame without an id.

How Has This Been Tested?

New tests in test/json_rpc_handler_test.rb cover non-object batch elements ([[]], [5], ["x"], [nil], [true]), a valid request followed by one non-object element, and handle_json("[[]]") returning an error instead of raising. A test in test/mcp/server/transports/stdio_transport_test.rb drives open with a [[]] line and asserts it emits an error response without raising. A test in test/mcp/client/stdio_test.rb has the server emit non-object frames before the response and asserts the client skips them and still returns the response.

Breaking Changes

None. A well-formed request, batch, or response is handled exactly as before; only a non-object message now yields a JSON-RPC error on the server or is skipped by the client instead of raising.

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