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.

lib/mcp/server_context.rb

@@ -30,6 +30,15 @@ module MCP
       @notification_target.notify_log_message(data: data, level: level, logger: logger, related_request_id: @related_request_id)
     end
 
+    # Sends a resource updated notification scoped to the originating session.
+    #
+    # @param uri [String] The URI of the updated resource.
+    def notify_resources_updated(uri:)
+      return unless @notification_target
+
+      @notification_target.notify_resources_updated(uri: uri)
+    end
+
     # Delegates to the session so the request is scoped to the originating client.
     def list_roots
       if @notification_target.respond_to?(:list_roots)