feat(df): add caching decorator for the policy checker

Second of three MRs splitting the former policy MR (!3722 (closed)).

Depends on !3744 (merged) (targets df-mr-03a-policy-core).

CachingChecker wraps any Checker and memoizes results by request key (coordinate + project + operation), so repeated checks for the same package during one package-manager run hit the underlying checker once.

• Keyed on coordinate and project and operation, since policies are project- and operation-scoped.
• Fails closed: if the inner checker errors, the request is treated as blocked and the error is not cached (it may be transient and retryable within the run).