Add Elicitation skeleton

The MCP 2025-06-18 specification includes a feature called Elicitation:
https://modelcontextprotocol.io/specification/2025-06-18/client/elicitation

Like Log Level, an implementation is expected to be provided in the near future.
This PR adds Elicitation as a skeleton implementation for now.

lib/mcp/methods.rb

@@ -21,6 +21,7 @@ module MCP
 
     ROOTS_LIST = "roots/list"
     SAMPLING_CREATE_MESSAGE = "sampling/createMessage"
+    ELICITATION_CREATE = "elicitation/create"
 
     # Notification methods
     NOTIFICATIONS_INITIALIZED = "notifications/initialized"
@@ -76,6 +77,8 @@ module MCP
           require_capability!(method, capabilities, :roots, :listChanged)
         when SAMPLING_CREATE_MESSAGE
           require_capability!(method, capabilities, :sampling)
+        when ELICITATION_CREATE
+          require_capability!(method, capabilities, :elicitation)
         when INITIALIZE, PING, NOTIFICATIONS_INITIALIZED, NOTIFICATIONS_PROGRESS, NOTIFICATIONS_CANCELLED
           # No specific capability required for initialize, ping, progress or cancelled
         end

lib/mcp/server.rb

@@ -80,6 +80,7 @@ module MCP
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) {},
         Methods::COMPLETION_COMPLETE => ->(_) {},
         Methods::LOGGING_SET_LEVEL => ->(_) {},
+        Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
     end