Fix JSON-RPC notification format in Streamable HTTP transport

Update send_notification method to properly construct JSON-RPC notification objects with jsonrpc version and method fields.
This ensures notifications conform to the JSON-RPC 2.0 specification when sent over HTTP transport.

https://www.jsonrpc.org/specification#notification

Another goal is to standardise the `send_notification` interface.
Before the revision, it was sufficient to pass the method and parameters in the case of stdio.
However, in the case of Streamable, a hash based on the JSON-RPC specification must be passed.

lib/mcp/server/transports/streamable_http_transport.rb

@@ -34,7 +34,13 @@ module MCP
           end
         end
 
-        def send_notification(notification, session_id: nil)
+        def send_notification(method, params = nil, session_id: nil)
+          notification = {
+            jsonrpc: "2.0",
+            method:,
+          }
+          notification[:params] = params if params
+
           @mutex.synchronize do
             if session_id
               # Send to specific session