fix: Remove Vitest workaround for client-side Astro component rendering
Removes the temporary workaround that allowed Vitest tests to render Astro components in a "client" environment.
This is a breaking change that requires users testing Astro component rendering with the container API in Vitest to use an SSR test environment (e.g., `node`) instead of client environments like `happy-dom` or `jsdom`.
Fixes #14895