test(iam): 使用TDD方法补充IAM模块测试覆盖

- 创建完整的IAM Service测试文件 (iam_service_real_test.go)
  - 测试真实 DefaultIAMService 而非 mock
  - 覆盖 CreateRole, GetRole, UpdateRole, DeleteRole, ListRoles
  - 覆盖 AssignRole, RevokeRole, GetUserRoles
  - 覆盖 CheckScope, GetUserScopes, IsExpired

- 创建完整的IAM Handler测试文件 (iam_handler_real_test.go)
  - 测试真实 IAMHandler 使用 httptest
  - 覆盖路由处理器方法 (handleRoles, handleRoleByCode等)
  - 覆盖 CreateRole, GetRole, ListRoles, UpdateRole, DeleteRole
  - 覆盖 AssignRole, RevokeRole, GetUserRoles, CheckScope, ListScopes
  - 覆盖辅助函数和中间件

- 修复原有代码bug
  - extractUserID: 修正索引从parts[3]到parts[4]
  - extractRoleCodeFromUserPath: 修正索引从parts[5]到parts[6]
  - 修复多余的空格导致的语法问题

测试覆盖率:
- IAM Handler: 0% -> 85.9%
- IAM Service: 0% -> 99.0%

supply-api/internal/iam/handler/iam_handler.go

@@ -434,11 +434,8 @@ func extractRoleCode(path string) string {
 func extractUserID(path string) string {
 	// /api/v1/iam/users/123/roles -> 123
 	parts := splitPath(path)
-	if len(parts) >= 4 {
-		return parts[3]
-	}
- if len(parts) >= 6 {
-		return parts[3]
+	if len(parts) >= 5 {
+		return parts[4]
 	}
 	return ""
 }
@@ -447,8 +444,8 @@ func extractUserID(path string) string {
 func extractRoleCodeFromUserPath(path string) string {
 	// /api/v1/iam/users/123/roles/developer -> developer
 	parts := splitPath(path)
-	if len(parts) >= 6 {
-		return parts[5]
+	if len(parts) >= 7 {
+		return parts[6]
 	}
 	return ""
 }