Support resource subscriptions per MCP specification

## Motivation and Context

The MCP specification defines `resources/subscribe`, `resources/unsubscribe`,
and `notifications/resources/updated` for clients to monitor resource changes:
https://modelcontextprotocol.io/specification/2025-03-26/server/resources#subscriptions

The Ruby SDK had stub (no-op) handlers but provided no way for server developers to
customize subscription behavior or send update notifications.

Following the Python SDK approach, the SDK does not track subscription state internally.
Server developers register handler blocks and manage their own subscription state,
allowing flexibility for different subscription semantics
(per-session tracking, persistence, debouncing, etc.).

Three methods are added:

- `Server#resources_subscribe_handler`: registers a handler for
  `resources/subscribe` requests
- `Server#resources_unsubscribe_handler`: registers a handler for
  `resources/unsubscribe` requests
- `ServerSession#notify_resources_updated`: sends a
  `notifications/resources/updated` notification to the subscribing client

`ServerContext#notify_resources_updated` is also added so that tool handlers
can send the notification scoped to the originating session.

## How Has This Been Tested?

All tests pass (`rake test`), RuboCop is clean. New tests cover custom handler registration
for `resources/subscribe` and `resources/unsubscribe`, session-scoped `notify_resources_updated` notifications,
error handling, and `ServerContext` delegation.

## Breaking Changes

None.

conformance/server.rb

@@ -2,6 +2,7 @@
 
 require "rackup"
 require "json"
+require "set"
 require "uri"
 require_relative "../lib/mcp"
 
@@ -539,6 +540,7 @@ module Conformance
       server.server_context = server
 
       configure_resources_read_handler(server)
+      configure_subscription_handlers(server)
       configure_completion_handler(server)
     end
 
@@ -609,6 +611,18 @@ module Conformance
       end
     end
 
+    def configure_subscription_handlers(server)
+      subscribed_uris = Set.new
+
+      server.resources_subscribe_handler do |params|
+        subscribed_uris.add(params[:uri].to_s)
+      end
+
+      server.resources_unsubscribe_handler do |params|
+        subscribed_uris.delete(params[:uri].to_s)
+      end
+    end
+
     def build_rack_app(transport)
       mcp_app = proc do |env|
         request = Rack::Request.new(env)