Use JSON-RPC error envelope for StreamableHTTPTransport errors

[koic]

Motivation and Context

PR #347 introduced a single JSON-RPC error envelope response for unsupported MCP-Protocol-Version headers, while the rest of the transport-level error responses (Accept, Content-Type, Invalid JSON, session management, method not allowed, internal server error) still returned plain JSON of the form { "error": "..." }. The Python SDK (src/mcp/server/streamable_http.py) and TypeScript SDK (packages/server/src/server/streamableHttp.ts) consistently use a JSON-RPC error envelope for all transport-level errors. Unify the Ruby SDK with them.

Behavior

All transport-level error responses now return a JSON-RPC error envelope:

{
  "jsonrpc": "2.0",
  "id": null,
  "error": {
    "code": -32600,
    "message": "..."
  }
}

Affected helpers in lib/mcp/server/transports/streamable_http_transport.rb:

• validate_content_type (HTTP 415)
• not_acceptable_response (HTTP 406)
• parse_request_body error (HTTP 400, PARSE_ERROR -32700)
• method_not_allowed_response (HTTP 405)
• missing_session_id_response (HTTP 400)
• session_not_found_response (HTTP 404)
• session_already_connected_response (HTTP 409)
• Internal server error fallback in handle_post (HTTP 500, INTERNAL_ERROR -32603)
• validate_protocol_version_header (HTTP 400, dedup with shared helper)

A new private helper json_rpc_error_response(status:, code:, message:) centralizes the envelope construction.

The "Invalid JSON" message wording is updated to "Parse error: Invalid JSON" to match the JSON-RPC 2.0 PARSE_ERROR semantics.

How Has This Been Tested?

Updated all affected tests in test/mcp/server/transports/streamable_http_transport_test.rb to assert the new envelope shape (body["error"]["message"] instead of body["error"]).

Breaking Changes

Clients that parsed the previous plain {"error": "..."} shape will need to read body["error"]["message"] (or body["error"]["code"]). The HTTP status codes are unchanged, only the response body structure changed.

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