Validate special characters in app_name config

[amikal04]

Summary of Changes

Implemented additional input validation in is_valid_app_name to reject app names containing invalid special characters (!@#$%^&*() etc.). Added corresponding unit tests in test_is_valid_app_name.py to ensure invalid names fail as expected.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool
  Assisted-by: Gemini

[freakboy3742]

I'm closing this on the basis that it is unclear why this is needed.

There's no issue number associated with the PR, and no explanation why this hardening is required - or why it does anything at all. The 4 provided test cases all behave correctly with the existing is_valid_app_name logic.