Stabilize password debounce test

[emil-e]

Summary

• make ghostel-test-password-debounce-defers-source-call capture the debounce timer instead of sleeping for wall-clock time
• manually invoke the captured callback to verify the deferred source path still runs once
• add comments documenting the test flow

Verification

• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-shell-test.el --eval "(ert-run-tests-batch-and-exit 'ghostel-test-password-debounce-defers-source-call)"
• full native ghostel-shell-test.el passed before the comment-only clarification