Introduce Capability Registry & Discovery

tests/Capability/Discovery/Fixtures/InvocableToolFixture.php

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+<?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\Tests\Capability\Discovery\Fixtures;
+
+use Mcp\Capability\Attribute\McpTool;
+
+#[McpTool(name: 'InvokableCalculator', description: 'An invokable calculator tool.')]
+class InvocableToolFixture
+{
+    /**
+     * Adds two numbers.
+     */
+    public function __invoke(int $a, int $b): int
+    {
+        return $a + $b;
+    }
+}