Fix trailing comment attribution when file has no final newline (#12082)

Fixes #12081.

The issue was the call to `MaybeDetachComment`: the conditional assumed that there was a next token, which was on the same line as the previous one, making attribution unclear. However, if there is no next token, we should not detach.

The actual fix is a one-liner. The rest of this PR is updates to the tests to verify this behavior under a handful of scenarios.

Closes #12082

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12082 from jhump:jh/fix-trailing-comment-attribution 767e41cb05ba6e176977e86fe33e9aad090f83f6
PiperOrigin-RevId: 513046172