* feat(s3): stub bucket configuration list endpoints
Adds Get and List handlers for Analytics, Inventory, IntelligentTiering,
and Metrics bucket configurations. List returns an empty result with
IsTruncated=false; single-get returns NoSuchConfiguration so SDK error
parsing remains predictable.
* review: gate stubs on bucket existence
All eight stub handlers now call checkBucket via stubBucketGuard so
NoSuchBucket takes precedence over NoSuchConfiguration / empty-list
responses, matching AWS S3 precedence. Tests provide a cached bucket
so the guard sees it as present.
cee2bf697c