[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/ClientCapabilities.php

@@ -20,7 +20,8 @@ namespace Mcp\Schema;
 class ClientCapabilities implements \JsonSerializable
 {
     /**
-     * @param array<string, mixed> $experimental
+     * @param array<string, mixed>  $experimental
+     * @param ?array<string, mixed> $extensions   protocol extensions the client supports (e.g. io.modelcontextprotocol/ui)
      */
     public function __construct(
         public readonly ?bool $roots = false,
@@ -28,6 +29,7 @@ class ClientCapabilities implements \JsonSerializable
         public readonly ?bool $sampling = null,
         public readonly ?bool $elicitation = null,
         public readonly ?array $experimental = null,
+        public readonly ?array $extensions = null,
     ) {
     }
 
@@ -39,6 +41,7 @@ class ClientCapabilities implements \JsonSerializable
      *     sampling?: bool,
      *     elicitation?: bool,
      *     experimental?: array<string, mixed>,
+     *     extensions?: array<string, mixed>,
      * } $data
      */
     public static function fromArray(array $data): self
@@ -68,7 +71,8 @@ class ClientCapabilities implements \JsonSerializable
             $rootsListChanged,
             $sampling,
             $elicitation,
-            $data['experimental'] ?? null
+            \is_array($data['experimental'] ?? null) ? $data['experimental'] : null,
+            \is_array($data['extensions'] ?? null) ? $data['extensions'] : null,
         );
     }
 
@@ -78,6 +82,7 @@ class ClientCapabilities implements \JsonSerializable
      *     sampling?: object,
      *     elicitation?: object,
      *     experimental?: object,
+     *     extensions?: object,
      * }|\stdClass
      */
     public function jsonSerialize(): array|object
@@ -102,6 +107,10 @@ class ClientCapabilities implements \JsonSerializable
             $data['experimental'] = (object) $this->experimental;
         }
 
+        if ($this->extensions) {
+            $data['extensions'] = (object) $this->extensions;
+        }
+
         return $data ?: new \stdClass();
     }
 }