[Schema][Server] Add MCP Apps extension (io.modelcontextprotocol/ui) support (#281)

* feat: add MCP Apps extension (io.modelcontextprotocol/ui) support

Adds support for the MCP Apps protocol extension, which lets servers
expose interactive HTML UIs as resources rendered by spec-conforming
clients (e.g. Goose) in sandboxed iframes.

- Generic extension plumbing: ServerExtensionInterface and
  Builder::enableExtension(), with extensions advertised under
  capabilities.extensions during the initialize handshake.
  ClientCapabilities/ServerCapabilities gain an extensions field.
- Schema DTOs under Mcp\Schema\Extension\Apps: the McpApps marker,
  UiToolMeta, ToolVisibility enum, UiResourceContentMeta,
  UiResourceCsp, and UiResourcePermissions (with empty-object
  presence markers per spec).
- A worked weather example under examples/server/mcp-apps/ with a
  minimal spec-conforming view (ui/initialize handshake, postMessage
  JSONRPCMessage objects, and ResizeObserver-driven size reporting).
- docs/extensions.md guide with pointers from index.md and
  examples.md to the ext-apps repository for the TypeScript SDK and
  richer view-side patterns.
- Unit and inspector snapshot test coverage.

Spec: https://github.com/modelcontextprotocol/ext-apps

* Type UiToolMeta visibility as ToolVisibility enum

Also fix UiResourceContentMeta::jsonSerialize() return phpdoc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Address review nitpicks for MCP Apps extension

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Apply PR review feedback for MCP Apps extension

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add regression tests and CHANGELOG entry for capabilities extensions

- Pin the is_array() guards on extensions in ServerCapabilities and
  ClientCapabilities::fromArray() with explicit non-array payload tests.
- Pin the isset()-rejects-null contract in UiResourcePermissions::fromArray().
- Surface the new extensions field on both capability classes in CHANGELOG.
- Inline the ?: idiom in Builder::build() to match the surrounding style.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Schema/Extension/ServerExtensionInterface.php

@@ -0,0 +1,40 @@
+<?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;
+
+/**
+ * A server-side 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>]` in the initialize
+ * response.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+interface ServerExtensionInterface
+{
+    /**
+     * 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>]` in the initialize response, so every value
+     * must be JSON-serializable (scalars, arrays, or `JsonSerializable` objects).
+     *
+     * @return array<string, mixed>
+     */
+    public function getCapabilities(): array;
+}