fix: force UTF-8 encoding for stdio streams

This change forces $stdin/$stdout to use UTF-8 encoding,
similar to PR 198 in the python-sdk.
https://github.com/modelcontextprotocol/python-sdk/pull/198

It addresses platform-dependent encoding issues.

lib/model_context_protocol/transports/stdio.rb

@@ -9,6 +9,8 @@ module ModelContextProtocol
       def initialize(server)
         @server = server
         @open = false
+        $stdin.set_encoding("UTF-8")
+        $stdout.set_encoding("UTF-8")
         super
       end