Release 1.4.0

This release repairs the `subscriptions/listen` stream: a host that buffers its responses,
the documented Rails controller pattern among them, can decline the method with
`serve_subscriptions_listen: false` instead of answering a modern client with a streaming body
it cannot render, and the stream's ordering guarantees are enforced - no notification before
the acknowledgement, none after the graceful closing result.

The new keyword argument is what makes this a minor release; everything else is a bug fix.
The controller-pattern crash is a regression from 1.2.0, where the modern lifecycle began
answering `subscriptions/listen` that earlier versions had refused as an unknown method,
and it fails quietly: only the notification stream breaks, so tool calls keep working.

lib/mcp/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module MCP
-  VERSION = "1.3.0"
+  VERSION = "1.4.0"
 end