fix: align mcp-preview AppBridge with ext-apps SDK v1.2.2 protocol

The AppBridge used pre-spec camelCase method names causing runtime errors
when widgets built with @modelcontextprotocol/ext-apps@1.2.2 called
app.sendMessage() or app.updateModelContext().

- Rename ui/sendMessage handler to ui/message (spec name), keep old name
  as backward-compat fallthrough
- Add ui/update-model-context stub handler (was falling through to
  default and throwing Unknown method)
- Update hostCapabilities: sendMessage true, updateModelContext {}
- Update bundled App.sendMessage() to send spec-correct "ui/message"
- Sync TypeScript source (app-bridge.ts, app.ts) with .mjs bundle
- Rewrite mcp-tester README as a focused landing page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: guyernest

crates/mcp-preview/assets/widget-runtime.mjs

@@ -1030,7 +1030,7 @@ var _App = class _App {
       return;
     }
     try {
-      await this._transport.send("ui/sendMessage", params);
+      await this._transport.send("ui/message", params);
     } catch (err) {
       console.warn("[App] sendMessage not supported by host:", err);
     }
@@ -1292,7 +1292,8 @@ var AppBridge = class {
           },
           hostCapabilities: {
             callTool: true,
-            sendMessage: false,
+            sendMessage: true,
+            updateModelContext: {},
             openLink: true
           },
           hostContext: this._hostContext
@@ -1305,9 +1306,13 @@ var AppBridge = class {
         }
         return await this._toolCallHandler(name, args);
       }
-      case "ui/sendMessage":
+      case "ui/message":
+      case "ui/sendMessage": // backward compat with pre-spec widgets
         console.log("[AppBridge] Widget sent message:", params);
         return {};
+      case "ui/update-model-context":
+        console.log("[AppBridge] Widget updated model context:", params);
+        return {};
       case "ui/openLink": {
         const url = params?.url;
         if (url) {