Extract server capabilities into dedicated Capabilities class

Refactor capabilities handling into MCP::Server::Capabilities to improve
code organization and separation of concerns. This provides a cleaner API
for configuring server capabilities including prompts, resources, tools,
completions, logging, and experimental features.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

lib/mcp/server/capabilities.rb

@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+module MCP
+  class Server
+    class Capabilities
+      def initialize(capabilities_hash = nil)
+        @completions = nil
+        @experimental = nil
+        @logging = nil
+        @prompts = nil
+        @resources = nil
+        @tools = nil
+
+        if capabilities_hash
+          support_completions if capabilities_hash.key?(:completions)
+          support_experimental(capabilities_hash[:experimental]) if capabilities_hash.key?(:experimental)
+          support_logging if capabilities_hash.key?(:logging)
+
+          if capabilities_hash.key?(:prompts)
+            support_prompts
+            prompts_config = capabilities_hash[:prompts] || {}
+            support_prompts_list_changed if prompts_config[:listChanged]
+          end
+
+          if capabilities_hash.key?(:resources)
+            support_resources
+            resources_config = capabilities_hash[:resources] || {}
+            support_resources_list_changed if resources_config[:listChanged]
+            support_resources_subscribe if resources_config[:subscribe]
+          end
+
+          if capabilities_hash.key?(:tools)
+            support_tools
+            tools_config = capabilities_hash[:tools] || {}
+            support_tools_list_changed if tools_config[:listChanged]
+          end
+        end
+      end
+
+      def support_completions
+        @completions ||= {}
+      end
+
+      def support_experimental(config = {})
+        @experimental = config || {}
+      end
+
+      def support_logging
+        @logging ||= {}
+      end
+
+      def support_prompts
+        @prompts ||= {}
+      end
+
+      def support_prompts_list_changed
+        support_prompts
+        @prompts[:listChanged] = true
+      end
+
+      def support_resources
+        @resources ||= {}
+      end
+
+      def support_resources_list_changed
+        support_resources
+        @resources[:listChanged] = true
+      end
+
+      def support_resources_subscribe
+        support_resources
+        @resources[:subscribe] = true
+      end
+
+      def support_tools
+        @tools ||= {}
+      end
+
+      def support_tools_list_changed
+        support_tools
+        @tools[:listChanged] = true
+      end
+
+      def to_h
+        {
+          completions: @completions,
+          experimental: @experimental,
+          logging: @logging,
+          prompts: @prompts,
+          resources: @resources,
+          tools: @tools,
+        }.compact
+      end
+    end
+  end
+end