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.

Author: koic

README.md

@@ -146,11 +146,12 @@ server.define_tool(name: "new_tool") { |**args| { result: "ok" } }
 server.notify_tools_list_changed
 ```
 
-### Unsupported Features ( to be implemented in future versions )
+### Unsupported Features (to be implemented in future versions)
 
 - Log Level
 - Resource subscriptions
 - Completions
+- Elicitation
 
 ### Usage
 

lib/mcp/methods.rb

@@ -21,6 +21,7 @@ module Methods
 
     ROOTS_LIST = "roots/list"
     SAMPLING_CREATE_MESSAGE = "sampling/createMessage"
+    ELICITATION_CREATE = "elicitation/create"
 
     # Notification methods
     NOTIFICATIONS_INITIALIZED = "notifications/initialized"
@@ -76,6 +77,8 @@ def ensure_capability!(method, capabilities)
           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 @@ def initialize(
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) {},
         Methods::COMPLETION_COMPLETE => ->(_) {},
         Methods::LOGGING_SET_LEVEL => ->(_) {},
+        Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
     end

test/mcp/methods_test.rb

@@ -74,6 +74,8 @@ def ensure_capability_does_not_raise_for(method, capabilities: {})
 
     ensure_capability_raises_error_for Methods::SAMPLING_CREATE_MESSAGE, required_capability_name: "sampling"
 
+    ensure_capability_raises_error_for Methods::ELICITATION_CREATE, required_capability_name: "elicitation"
+
     # Methods and notifications of both server and client
     ensure_capability_does_not_raise_for Methods::PING
     ensure_capability_does_not_raise_for Methods::NOTIFICATIONS_PROGRESS