[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/SeatCompletionProvider.php

@@ -0,0 +1,42 @@
+<?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\Completion\ProviderInterface;
+
+/**
+ * A provider that cannot be built without its seat map.
+ *
+ * The constructor takes a scalar the auto-wiring container cannot supply, so a
+ * completion that comes back with seats in it proves the provider was taken
+ * from the container rather than instantiated on the spot.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class SeatCompletionProvider implements ProviderInterface
+{
+    /**
+     * @param list<string> $seats
+     */
+    public function __construct(
+        private readonly array $seats,
+    ) {
+    }
+
+    public function getCompletions(string $currentValue): array
+    {
+        return array_values(array_filter(
+            $this->seats,
+            static fn (string $seat): bool => str_starts_with($seat, $currentValue),
+        ));
+    }
+}