Keep Ruby 2.7 from warning about keywords in StdioTransport

Since 1.8.0, `ServerSession#handle` accepts bare keyword arguments as the request (`handle(request = nil, auth_info: nil, **request_keywords)`),
so `StdioTransport` passing the parsed request Hash as its only argument makes Ruby 2.7 treat the Hash as keyword arguments.
The request is still handled, but every frame prints
"Using the last argument as keyword parameters is deprecated" to stderr when deprecation warnings are enabled,
which breaks callers that expect a silent stderr, such as RuboCop's MCP server specs on Ruby 2.7.

Passing `auth_info: nil` explicitly keeps the Hash positional without changing the request that reaches the server.

Author: koic

lib/mcp/server/transports/stdio_transport.rb

@@ -168,7 +168,9 @@ def parse_line(line)
         # (an `initialize` after a modern lock, a modern envelope after a legacy lock, or a missing envelope after a modern lock)
         # are rejected in-band by `Server#lift_request_envelope`.
         def dispatch_with_era(parsed)
-          response = @session.handle(parsed)
+          # Passing `auth_info:` keeps Ruby 2.7 from taking the request Hash as keyword arguments,
+          # which works but warns that the conversion is deprecated.
+          response = @session.handle(parsed, auth_info: nil)
           lock_modern_era_on_success(parsed, response)
           response
         end

test/mcp/server/transports/stdio_transport_test.rb

@@ -135,6 +135,25 @@ class StdioTransportTest < ActiveSupport::TestCase
           end
         end
 
+        test "open passes a request to the session without a keyword argument warning" do
+          request = { jsonrpc: "2.0", method: "ping", id: "1" }
+          input = StringIO.new(JSON.generate(request) + "\n")
+          original_stdin = $stdin
+          original_deprecated = Warning[:deprecated]
+
+          begin
+            $stdin = input
+            Warning[:deprecated] = true
+
+            _out, err = capture_io { @transport.open }
+
+            assert_empty(err)
+          ensure
+            $stdin = original_stdin
+            Warning[:deprecated] = original_deprecated
+          end
+        end
+
         test "open handles a malformed JSON-RPC batch element and emits an error response" do
           input = StringIO.new("[[]]\n")
           output = StringIO.new