Previously, evaluateIAMPolicies created a new PolicyEngine and re-parsed
the JSON policy document for every policy on every request. This adds a
shared iamPolicyEngine field that caches compiled policies, kept in sync
by PutPolicy, DeletePolicy, and bulk config reload paths.
- PutPolicy deletes the old cache entry before setting the new one, so a
parse failure on update does not leave a stale allow.
- Log warnings when policy compilation fails instead of silently
discarding errors.
- Add test for valid-to-invalid policy update regression.
1b6e96614d