[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/BookingElements.php

@@ -0,0 +1,38 @@
+<?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\Integration\Fixture\Completion;
+
+use Mcp\Capability\Attribute\CompletionProvider;
+use Mcp\Capability\Attribute\McpPrompt;
+
+/**
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class BookingElements
+{
+    /**
+     * Confirms a seat booking.
+     *
+     * @param string $seat the seat to book
+     *
+     * @return array the prompt messages
+     */
+    #[McpPrompt(name: 'book_seat')]
+    public function bookSeat(
+        #[CompletionProvider(provider: SeatCompletionProvider::class)]
+        string $seat,
+    ): array {
+        return [
+            ['role' => 'user', 'content' => \sprintf('Book seat %s for me.', $seat)],
+        ];
+    }
+}