refactor: replace array_merge with array union for cleaner CORS headers initialization

Co-authored-by: Antoine Bluchet <[email protected]>

Author: CodeWithKyrian

src/Server/Transport/StreamableHttpTransport.php

@@ -56,11 +56,11 @@ public function __construct(
         $sessionIdString = $this->request->getHeaderLine('Mcp-Session-Id');
         $this->sessionId = $sessionIdString ? Uuid::fromString($sessionIdString) : null;
 
-        $this->corsHeaders = array_merge([
+        $this->corsHeaders = $corsHeaders + [
             'Access-Control-Allow-Origin' => '*',
             'Access-Control-Allow-Methods' => 'GET, POST, DELETE, OPTIONS',
             'Access-Control-Allow-Headers' => 'Content-Type, Mcp-Session-Id, Mcp-Protocol-Version, Last-Event-ID, Authorization, Accept',
-        ], $corsHeaders);
+        ];
     }
 
     public function initialize(): void