[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>

examples/server/mcp-apps/WeatherApp.php

@@ -0,0 +1,67 @@
+<?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\Example\Server\McpApps;
+
+use Mcp\Schema\Content\TextResourceContents;
+use Mcp\Schema\Extension\Apps\McpApps;
+use Mcp\Schema\Extension\Apps\UiResourceContentMeta;
+use Mcp\Schema\Extension\Apps\UiResourceCsp;
+use Mcp\Schema\Extension\Apps\UiResourcePermissions;
+
+final class WeatherApp
+{
+    public function getWeatherApp(): TextResourceContents
+    {
+        $contentMeta = new UiResourceContentMeta(
+            csp: new UiResourceCsp(
+                connectDomains: ['https://api.weather.example.com'],
+            ),
+            permissions: new UiResourcePermissions(
+                geolocation: true,
+            ),
+            prefersBorder: true,
+        );
+
+        return new TextResourceContents(
+            uri: 'ui://weather-app',
+            mimeType: McpApps::MIME_TYPE,
+            text: file_get_contents(__DIR__.'/weather-app.html'),
+            meta: ['ui' => $contentMeta],
+        );
+    }
+
+    public function getWeather(string $city): string
+    {
+        $weather = [
+            'london' => ['temp' => '15°C', 'condition' => 'Cloudy', 'humidity' => '78%'],
+            'paris' => ['temp' => '18°C', 'condition' => 'Sunny', 'humidity' => '55%'],
+            'tokyo' => ['temp' => '22°C', 'condition' => 'Partly Cloudy', 'humidity' => '65%'],
+            'new york' => ['temp' => '12°C', 'condition' => 'Rainy', 'humidity' => '85%'],
+            'lagos' => ['temp' => '30°C', 'condition' => 'Sunny', 'humidity' => '82%'],
+            'stockholm' => ['temp' => '4°C', 'condition' => 'Cloudy', 'humidity' => '70%'],
+            'berlin' => ['temp' => '9°C', 'condition' => 'Partly Cloudy', 'humidity' => '68%'],
+            'sydney' => ['temp' => '26°C', 'condition' => 'Sunny', 'humidity' => '60%'],
+            'buenos aires' => ['temp' => '24°C', 'condition' => 'Rainy', 'humidity' => '80%'],
+        ];
+
+        $key = strtolower($city);
+        $data = $weather[$key] ?? ['temp' => '20°C', 'condition' => 'Clear', 'humidity' => '60%'];
+
+        return \sprintf(
+            'Weather in %s: %s, %s, Humidity: %s',
+            $city,
+            $data['temp'],
+            $data['condition'],
+            $data['humidity'],
+        );
+    }
+}