[Client] add client conformance test suite (#279)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Schema/ClientCapabilities.php

@@ -78,9 +78,9 @@ class ClientCapabilities implements \JsonSerializable
      *     sampling?: object,
      *     elicitation?: object,
      *     experimental?: object,
-     * }
+     * }|\stdClass
      */
-    public function jsonSerialize(): array
+    public function jsonSerialize(): array|object
     {
         $data = [];
         if ($this->roots || $this->rootsListChanged) {
@@ -102,6 +102,6 @@ class ClientCapabilities implements \JsonSerializable
             $data['experimental'] = (object) $this->experimental;
         }
 
-        return $data;
+        return $data ?: new \stdClass();
     }
 }