[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>

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-prompts_list.json

@@ -0,0 +1,3 @@
+{
+  "prompts": []
+}

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-resources_list.json

@@ -0,0 +1,13 @@
+{
+  "resources": [
+    {
+      "name": "weather-app",
+      "uri": "ui://weather-app",
+      "description": "Interactive weather dashboard",
+      "mimeType": "text/html;profile=mcp-app",
+      "_meta": {
+        "ui": {}
+      }
+    }
+  ]
+}

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-resources_templates_list.json

@@ -0,0 +1,3 @@
+{
+  "resourceTemplates": []
+}

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-tools_call-get_weather_london.json

@@ -0,0 +1,9 @@
+{
+  "content": [
+    {
+      "type": "text",
+      "text": "Weather in London: 15°C, Cloudy, Humidity: 78%"
+    }
+  ],
+  "isError": false
+}

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-tools_call-get_weather_tokyo.json

@@ -0,0 +1,9 @@
+{
+  "content": [
+    {
+      "type": "text",
+      "text": "Weather in Tokyo: 22°C, Partly Cloudy, Humidity: 65%"
+    }
+  ],
+  "isError": false
+}

tests/Inspector/Stdio/snapshots/StdioMcpAppsTest-tools_list.json

@@ -0,0 +1,28 @@
+{
+  "tools": [
+    {
+      "name": "get_weather",
+      "description": "Get current weather for a city",
+      "inputSchema": {
+        "type": "object",
+        "properties": {
+          "city": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "city"
+        ]
+      },
+      "_meta": {
+        "ui": {
+          "resourceUri": "ui://weather-app",
+          "visibility": [
+            "model",
+            "app"
+          ]
+        }
+      }
+    }
+  ]
+}