feat: add request-state key rotation (#1128)

* feat: add request-state key rotation

* docs: streamline request-state codec documentation

* fix: harden request-state fallback verification

* refactor: refine request-state keyring API

* docs: make request-state rotation guidance self-contained

* docs: streamline request-state keyring rustdocs

* test: streamline request-state keyring coverage

* docs: restore request-state key rotation doctest

* chore: remove manual changelog entry