[Server] Serve an extension's methods under the modern lifecycle (#454)

* [Server] Serve an extension's methods under the modern lifecycle

The modern dispatcher takes the method-to-extension map the builder collects,
so a method belonging to an extension this server does not serve is answered
-32601 naming the extension instead of a bare "no handler found". It is still
an unknown method - the server genuinely does not implement it - but the
caller can now act on the answer.

* [Server] Reject two extensions claiming the same RPC method

The message factory resolves a contested method to whichever class
registered first, while extensionMethods kept the last one — so error
messages could name the wrong extension. Also fixes a test that
claimed to prove a method gets named by its extension while asserting
the opposite; the case it meant to cover (an enabled extension with no
handler for one of its methods) had no coverage at all.

tests/Unit/Server/Extension/UnservedThingExtension.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\Unit\Server\Extension;
+
+use Mcp\Schema\Extension\ExtensionIdentifier;
+use Mcp\Schema\Extension\ExtensionInterface;
+
+/**
+ * An extension that declares a method it does not serve: {@see ThingListRequest}
+ * is registered so the method decodes, but no handler answers it.
+ */
+final class UnservedThingExtension implements ExtensionInterface
+{
+    public function getId(): ExtensionIdentifier
+    {
+        return new ExtensionIdentifier('com.example/unserved-things');
+    }
+
+    public function getCapabilities(): array
+    {
+        return [];
+    }
+
+    public function getMessages(): array
+    {
+        return [ThingListRequest::class];
+    }
+
+    public function getRequestHandlers(): iterable
+    {
+        return [];
+    }
+}