Introduce Capability Registry & Discovery

tests/Capability/Discovery/Fixtures/InvocableResourceTemplateFixture.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\McpResourceTemplate;
+
+#[McpResourceTemplate(uriTemplate: 'invokable://user-profile/{userId}')]
+class InvocableResourceTemplateFixture
+{
+    public function __invoke(string $userId): array
+    {
+        return ['id' => $userId, 'email' => "user{$userId}@example-invokable.com"];
+    }
+}