Complete "Giving errors back to model" pattern documentation

Adds JSDoc example `App_updateModelContext_reportError` showing how to
report runtime errors (e.g., microphone permission denied) back to the
model using `updateModelContext`.

Updates patterns.md with clearer framing:
- Server-side (tool handler): return `{ isError: true, ... }`
- Client-side (app): use `updateModelContext` for runtime errors

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.ts

@@ -674,6 +674,9 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
    * @example Update with structured content
    * {@includeCode ./app.examples.ts#App_updateModelContext_structuredContent}
    *
+   * @example Report runtime error to model
+   * {@includeCode ./app.examples.ts#App_updateModelContext_reportError}
+   *
    * @returns Promise that resolves when the context update is acknowledged
    */
   updateModelContext(