* feat(iam): implement ListUserPolicies API action (#8987)
Add ListUserPolicies support to both embedded and standalone IAM servers,
resolving the NotImplemented error when calling `aws iam list-user-policies`.
* fix: address review feedback for ListUserPolicies
- Add handleImplicitUsername for ListUserPolicies in both IAM servers
so omitting UserName defaults to the calling user (Gemini review)
- Assert synthetic policy name in unit test (CodeRabbit)
- Use require.True for error type assertion in integration test (CodeRabbit)