[Schema][Client][Server] Close MCP Apps gaps: extension negotiation checks (#429)

* [Schema][Client][Server] Close MCP Apps gaps: extension negotiation checks

Add ClientGateway::supportsExtension(), Client\Builder::enableExtension() and
ClientCapabilities::withExtensions(); introduce side-agnostic ExtensionInterface
and deprecate ServerExtensionInterface; document attribute-based UI linkage.

* Drop ServerExtensionInterface in favour of ExtensionInterface

src/Schema/Extension/ExtensionInterface.php

@@ -0,0 +1,41 @@
+<?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\Schema\Extension;
+
+/**
+ * An MCP protocol extension advertised during capability negotiation.
+ *
+ * Implementations are typically zero-config — they expose a stable identifier and the
+ * capability payload announced under `capabilities.extensions[<id>]`. The same
+ * extension object can be enabled on a server (initialize response) and on a client
+ * (initialize request); the side that enables it decides which.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+interface ExtensionInterface
+{
+    /**
+     * The reverse-DNS identifier used as the key under `capabilities.extensions`.
+     */
+    public function getId(): string;
+
+    /**
+     * The capability payload announced for this extension.
+     *
+     * The returned array is cast to an object and embedded under
+     * `capabilities.extensions[<id>]`, so every value must be JSON-serializable
+     * (scalars, arrays, or `JsonSerializable` objects).
+     *
+     * @return array<string, mixed>
+     */
+    public function getCapabilities(): array;
+}