ci(lit): Do not modify a2ui messages in testing harness

[ditman]

Description

While setting up the integration tests for React, I found some weird failures with some test cases that pointed to A2UI Messages being modified by the testing harness as tests executed.

The solution is to deep clone the test case messages when a test is initialized so they're always "fresh". See it in angular here.

The problem also exists on the Lit testing harness, but it was being masked by the tests doing some sneaky initialization.

This PR removes the sneaky initialization of tests, and clones the messages so they pass as expected.

(It also adds a dependency on prettier and ensures that yarn format works, something that I found broken while attempting to format my changes before creating this PR)

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request updates the Prettier configuration in the package.json files of both renderers/lit and renderers/lit/a2ui_explorer to reference a shared ignore path, and adds Prettier as a development dependency. Additionally, it wraps modified messages in structuredClone before processing in local-gallery.ts, and introduces initial value assertions for inputs in the live invitation builder tests. There are no review comments, and we have no additional feedback to provide.