[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/CompletionTest.php

@@ -0,0 +1,57 @@
+<?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;
+
+use Mcp\Schema\PromptReference;
+use PHPUnit\Framework\Attributes\TestDox;
+
+/**
+ * Argument completion driven by a `#[CompletionProvider(provider: …)]` class-string.
+ *
+ * The fixture's provider only exists in the container, so completions coming
+ * back at all is what proves the attribute reached the registry and the
+ * container was asked to build it.
+ *
+ * @see Fixture/completion.php for the server under test
+ */
+final class CompletionTest extends IntegrationTestCase
+{
+    #[TestDox('a class-string provider completes from the container-built instance')]
+    public function testClassStringProviderCompletesFromTheContainer(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'seat', 'value' => '12']);
+
+        $this->assertSame(['12A', '12B'], $result->values);
+    }
+
+    #[TestDox('an empty value offers every completion the provider knows')]
+    public function testEmptyValueOffersEveryCompletion(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'seat', 'value' => '']);
+
+        $this->assertSame(['12A', '12B', '14C'], $result->values);
+    }
+
+    #[TestDox('an argument the prompt does not declare completes to nothing')]
+    public function testUnknownArgumentCompletesToNothing(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'unknown', 'value' => '1']);
+
+        $this->assertSame([], $result->values);
+    }
+}