Creates tests for Hilla use cases on both sides of the browser boundary: Vitest + React Testing Library tests for the React/TypeScript view (with the generated endpoint clients mocked) and Spring Boot integration tests for the @BrowserCallable service behind it. Use when the user asks to "test a Hilla view", "write Hilla tests", "test a React view for Vaadin", "test a @BrowserCallable service", "write Vitest tests for a Hilla app", or mentions Hilla testing, React Testing Library for Vaadin, endpoint mocking, or testing TSX views.