Introduce Capability Registry & Discovery

tests/Capability/Attribute/CompletionProviderFixture.php

@@ -0,0 +1,30 @@
+<?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\Attribute;
+
+use Mcp\Capability\Prompt\Completion\ProviderInterface;
+
+class CompletionProviderFixture implements ProviderInterface
+{
+    /**
+     * @var string[]
+     */
+    public static array $completions = ['alpha', 'beta', 'gamma'];
+    public static string $lastCurrentValue = '';
+
+    public function getCompletions(string $currentValue): array
+    {
+        self::$lastCurrentValue = $currentValue;
+
+        return array_filter(self::$completions, fn ($item) => str_starts_with($item, $currentValue));
+    }
+}