Fix FluentMenu service disposal (#5032) (#5035)

Unregister service-backed menus during disposal and notify the provider after successful removal. Add lifecycle regression coverage for registration, idempotency, and duplicate IDs.


Copilot-Session: 8541ad78-5e1c-4374-a3a5-7482a945eb69

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>