Reject a bundle identifier with a trailing newline

[rawsun007]

is_valid_bundle_identifier applies VALID_BUNDLE_RE with re.match, and $ matches before a final newline, so a bundle ending in one validates:

>>> is_valid_bundle_identifier("com.example\n")
True

The value then reaches the places that use the identifier as a name — flatpak.py joins it onto the build path, and appimage.py puts it in a Docker image tag:

>>> app = AppConfig(app_name="myapp", version="1.0", bundle="com.example\n", ...)
>>> app.bundle_identifier
'com.example\n.myapp'

fullmatch is what is_valid_pep508_name two hundred lines above already uses, and it makes the trailing $ redundant, so that comes off too. Reverting only config.py fails exactly the one new case.

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: Claude Opus 5 (Claude Code)

[mhsmith]

Thanks!