Handle 202 Accepted response in HTTP client

Per the Streamable HTTP spec, a server returns 202 Accepted with no
body as the ACK for a JSON-RPC notification or response. The client
previously errored on 202 because it fell through Content-Type
dispatch. Short-circuit to return nil before Content-Type handling so
fire-and-forget messages (e.g. notifications/initialized) no longer
raise.

https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/mcp/client/http.rb

@@ -100,6 +100,10 @@ module MCP
       end
 
       def parse_response_body(response, method, params)
+        # 202 Accepted is the server's ACK for a JSON-RPC notification or response; no body is expected.
+        # https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server
+        return if response.status == 202
+
         content_type = response.headers["Content-Type"]
 
         if content_type&.include?("text/event-stream")