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.rb

@@ -117,6 +117,8 @@ module MCP
         Methods::RESOURCES_LIST => method(:list_resources),
         Methods::RESOURCES_READ => method(:read_resource_no_content),
         Methods::RESOURCES_TEMPLATES_LIST => method(:list_resource_templates),
+        Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
+        Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
         Methods::TOOLS_LIST => method(:list_tools),
         Methods::TOOLS_CALL => method(:call_tool),
         Methods::PROMPTS_LIST => method(:list_prompts),
@@ -128,10 +130,6 @@ module MCP
         Methods::NOTIFICATIONS_ROOTS_LIST_CHANGED => ->(_) {},
         Methods::COMPLETION_COMPLETE => ->(_) { DEFAULT_COMPLETION_RESULT },
         Methods::LOGGING_SET_LEVEL => method(:configure_logging_level),
-
-        # No op handlers for currently unsupported methods
-        Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
-        Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
       }
       @transport = transport
     end
@@ -258,6 +256,24 @@ module MCP
       @handlers[Methods::COMPLETION_COMPLETE] = block
     end
 
+    # Sets a custom handler for `resources/subscribe` requests.
+    # The block receives the parsed request params. The return value is
+    # ignored; the response is always an empty result `{}` per the MCP specification.
+    #
+    # @yield [params] The request params containing `:uri`.
+    def resources_subscribe_handler(&block)
+      @handlers[Methods::RESOURCES_SUBSCRIBE] = block
+    end
+
+    # Sets a custom handler for `resources/unsubscribe` requests.
+    # The block receives the parsed request params. The return value is
+    # ignored; the response is always an empty result `{}` per the MCP specification.
+    #
+    # @yield [params] The request params containing `:uri`.
+    def resources_unsubscribe_handler(&block)
+      @handlers[Methods::RESOURCES_UNSUBSCRIBE] = block
+    end
+
     def build_sampling_params(
       capabilities,
       messages:,
@@ -391,6 +407,9 @@ module MCP
             init(params, session: session)
           when Methods::RESOURCES_READ
             { contents: @handlers[Methods::RESOURCES_READ].call(params) }
+          when Methods::RESOURCES_SUBSCRIBE, Methods::RESOURCES_UNSUBSCRIBE
+            @handlers[method].call(params)
+            {}
           when Methods::TOOLS_CALL
             call_tool(params, session: session, related_request_id: related_request_id)
           when Methods::COMPLETION_COMPLETE