test: cover studio plugin registration

[benbenlijie]

/claim #71

Summary

• Add focused Vitest coverage for StudioPlugin registration behavior.
• Verify the plugin exposes the expected unauthenticated studio metadata and default /studio prefix.
• Verify a custom route is registered and the registered callback serves the embedded Studio HTML when Basic auth matches.
• Verify an empty prefix disables route registration.

This is intentionally scoped to plugins/studio/index.test.ts and separate from the existing Studio handler auth coverage work.

Validation

• corepack pnpm exec vitest run plugins/studio/index.test.ts
• corepack pnpm exec prettier --check plugins/studio/index.test.ts