Add elicitation support for server-to-client user input requests (#228)

- Add ElicitAction enum (accept/decline/cancel)
- Add schema definitions for elicitation fields (string, number, boolean, enum)
- Add ElicitationSchema wrapper for requestedSchema
- Add ElicitRequest and ElicitResult for elicitation/create method
- Update ClientCapabilities to support elicitation capability
- Add elicit() method to ClientGateway
- Store client capabilities in session during initialization
- Add comprehensive unit tests
- Add elicitation example server with book_restaurant, confirm_action, and collect_feedback tools

src/Schema/ClientCapabilities.php

@@ -26,6 +26,7 @@ class ClientCapabilities implements \JsonSerializable
         public readonly ?bool $roots = false,
         public readonly ?bool $rootsListChanged = null,
         public readonly ?bool $sampling = null,
+        public readonly ?bool $elicitation = null,
         public readonly ?array $experimental = null,
     ) {
     }
@@ -36,6 +37,7 @@ class ClientCapabilities implements \JsonSerializable
      *         listChanged?: bool,
      *     },
      *     sampling?: bool,
+     *     elicitation?: bool,
      *     experimental?: array<string, mixed>,
      * } $data
      */
@@ -56,10 +58,16 @@ class ClientCapabilities implements \JsonSerializable
             $sampling = true;
         }
 
+        $elicitation = null;
+        if (isset($data['elicitation'])) {
+            $elicitation = true;
+        }
+
         return new self(
             $rootsEnabled,
             $rootsListChanged,
             $sampling,
+            $elicitation,
             $data['experimental'] ?? null
         );
     }
@@ -68,6 +76,7 @@ class ClientCapabilities implements \JsonSerializable
      * @return array{
      *     roots?: object,
      *     sampling?: object,
+     *     elicitation?: object,
      *     experimental?: object,
      * }
      */
@@ -85,6 +94,10 @@ class ClientCapabilities implements \JsonSerializable
             $data['sampling'] = new \stdClass();
         }
 
+        if ($this->elicitation) {
+            $data['elicitation'] = new \stdClass();
+        }
+
         if ($this->experimental) {
             $data['experimental'] = (object) $this->experimental;
         }