Remove contacts sample in the lit shell app

[josemontespg]

Related to #1113

What was done

• Cleaned up Lit shell contacts by removing contact-related configurations and scripts.
• Addressed review feedback: Added a snackbar warning message when the app query param does not match any available config (e.g., ?app=contacts), falling back gracefully to the Restaurant Finder.
• Fixed a lifecycle timing issue in the Lit shell to ensure queued snackbar messages are displayed after the component is fully rendered.

Why

• This is Part of the plan to delete the contact sample.
• To keep the main repository clean and focused on code.
• To provide a better user experience when accessing removed or invalid sample applications.

Literal Plan Used

# Plan: Cleanup Lit Shell Contacts (Part 1)

This plan covers the removal of contact-related configurations from the Lit shell.

## Git Branch
`feature/cleanup-lit-shell-contacts`

## Files to Delete
- `samples/client/lit/shell/configs/contacts.ts`

## Files to Update
- `samples/client/lit/shell/app.ts`: Update to remove import of `configs/contacts.ts` and set default to `restaurant`.
  - *Important*: Perform this update *before* deleting `configs/contacts.ts` to avoid breaking the build.
- `samples/client/lit/shell/package.json`: Remove `"demo:contact"` script.
- `samples/client/lit/package.json`: Remove `"serve:agent:contact_lookup"` and `"demo:contact"` scripts.

## Validation Steps

### Build Validation
- Verify that the Lit shell still builds.
  - Command: `cd samples/client/lit/shell && npm run build`

Screenshot

Showing the warning snackbar when the app query param does not correspond to an app in the config

[gemini-code-assist]

Code Review

This pull request removes the Contact Manager sample application, including its configuration, scripts, and associated imports, while setting the restaurant finder as the new default. It also updates the @a2ui/lit version and introduces an optional peer dependency for markdown. Review feedback identifies a TypeScript version inconsistency across the monorepo and suggests a more graceful handling for users attempting to access the removed contacts application via URL parameters.