ci: validate codegen SUITE_EXCLUSIONS independently of source scope #8266

scripts/ci_e2e_scope.py structurally validates every exclusion on every run, but the behavioral exact-test checks execute only when the selected suite set contains perry-codegen. A fix that changes HIR, transform, or another dependency without selecting codegen can make an excluded assertion pass while leaving the exclusion in place. Add a dedicated validation job or a dependency-aware trigger without forcing the full e2e toolchain onto docs-only PRs. Raised from the final review of #8208; that PR documents the current scope rather than claiming global self-invalidation.