[Server] Reorganize server handlers and transport structure for better organization (#86)

* refactor: reorganize server handlers and transport structure

* feat: move the test files to corresponding directories

* fix: linter issues

* fix: correct PHPDoc formatting and streamline handler imports

* refactor: update import statement for ServerBuilder to Builder

src/Schema/Page.php

@@ -0,0 +1,35 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Schema;
+
+/**
+ * @phpstan-type PageItem Tool|Prompt|ResourceTemplate|Resource
+ *
+ * @extends \ArrayObject<int|string, PageItem>
+ */
+final class Page extends \ArrayObject
+{
+    /**
+     * @param array<int|string, PageItem> $references Items can be Tool, Prompt, ResourceTemplate, or Resource
+     */
+    public function __construct(
+        public readonly array $references,
+        public readonly ?string $nextCursor,
+    ) {
+        parent::__construct($references, \ArrayObject::ARRAY_AS_PROPS);
+    }
+
+    public function count(): int
+    {
+        return \count($this->references);
+    }
+}