Add logging support

A server can send structured logging messages to the client.
https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging#logging

Logging was specified in the 2024-11-05 specification, but since it was not supported in ruby-sdk, I implemented it.
https://modelcontextprotocol.io/specification/2024-11-05/server/utilities/logging

I also made it possible to output a simple notification message in the examples.

examples/streamable_http_client.rb

@@ -122,6 +122,10 @@ def main
     exit(1)
   end
 
+  if init_response[:body].dig("result", "capabilities", "logging")
+    make_request(session_id, "logging/setLevel", { level: "info" })
+  end
+
   logger.info("Session initialized: #{session_id}")
   logger.info("Server info: #{init_response[:body]["result"]["serverInfo"]}")
 

examples/streamable_http_server.rb

@@ -107,6 +107,7 @@ app = proc do |env|
         mcp_logger.error("Response error: #{parsed_response["error"]["message"]}")
       elsif parsed_response["accepted"]
         # Response was sent via SSE
+        server.notify_log_message(data: { details: "Response accepted and sent via SSE" }, level: "info")
         sse_logger.info("Response sent via SSE stream")
       else
         mcp_logger.info("Response: success (id: #{parsed_response["id"]})")