feat/Issue-13: elicitation support (#188)

fixes: #13 

 Implements elicitation functionality from MCP 2025-06-18 specification.

 Changes
 - Add ElicitParams and ElicitResult protocol types 
- Add ServerSession.Elicit() method and ClientOptions.ElicitationHandler
 - Schema validation enforces top-level properties only
 - Support for accept/decline/cancel actions with progress tokens

Tests
 - Integration test in TestEndToEnd
 - Schema validation, error handling, and capability declaration tests

mcp/server.go

@@ -834,6 +834,11 @@ func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessag
 	return handleSend[*CreateMessageResult](ctx, methodCreateMessage, newServerRequest(ss, orZero[Params](params)))
 }
 
+// Elicit sends an elicitation request to the client asking for user input.
+func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*ElicitResult, error) {
+	return handleSend[*ElicitResult](ctx, methodElicit, newServerRequest(ss, orZero[Params](params)))
+}
+
 // Log sends a log message to the client.
 // The message is not sent if the client has not called SetLevel, or if its level
 // is below that of the last SetLevel.