[Capability] Remove CompletionProvider's dead providerClass argument (#499)

* [Capability] Remove CompletionProvider's dead providerClass argument

It always threw on construction, so it never worked in any release. `provider:`
takes the same class-string and is now the first positional argument.

* Reword Changelog

tests/Integration/Fixture/completion.php

@@ -0,0 +1,31 @@
+<?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.
+ */
+
+/*
+ * Server for {@see \Mcp\Tests\Integration\CompletionTest}.
+ */
+
+use Mcp\Capability\Registry\Container;
+use Mcp\Server;
+use Mcp\Server\Transport\StdioTransport;
+use Mcp\Tests\Integration\Fixture\Completion\SeatCompletionProvider;
+
+require_once dirname(__DIR__, 3).'/vendor/autoload.php';
+
+$container = new Container();
+$container->set(SeatCompletionProvider::class, new SeatCompletionProvider(['12A', '12B', '14C']));
+
+Server::builder()
+    ->setServerInfo('integration-server', '1.0.0')
+    ->setContainer($container)
+    ->setDiscovery(__DIR__, ['Completion'])
+    ->build()
+    ->run(new StdioTransport());