Minimize review-request noise for backend maintainers

Avoid requesting backend-maintainer reviews when changes only touch backend-owned paths incidentally and do not meaningfully alter backend behavior. In particular, for dependency-graph pruning, package moves, test-helper isolation, or source-address normalization that merely affects indirect backend dependencies, remove automatically added CODEOWNERS review requests or otherwise limit unnecessary notifications. Request backend review only when the change has substantive backend behavior or ownership implications.