* fix(iam): return a valid user ARN from CreateUser and GetUser
The terraform aws provider 6.41 reads a user back after creating it and
blocks until GetUser returns a value that passes arn.IsARN. We only set
UserName, so the ARN was empty and apply hung until the 2m timeout.
Populate Arn (and Path) via a shared iam.NewUser helper in both the
embedded and standalone IAM handlers.
* fix(iam): use the userName parameter directly in NewUser
Drop the redundant local copy; the value parameter is already function-local.
* fix(iam): return full user objects with ARNs from GetGroup
GetGroup listed members with only UserName set. Build them via the shared
NewUser helper so group members carry a valid Arn and Path like the other
user responses, in both the embedded and standalone IAM handlers.