Creates a new user.

Required fields: FirstName, LastName, LoginName, GroupIDs (must contain at least one ID). Optional fields: PhoneNumber, CorrespondenceEmailAddress, BillingAccess, EnableSAMLDirectLogin, Enabled, DisableOnDate.

Notes:

• If CorrespondenceEmailAddress is not provided, LoginName is used for correspondence.
• If Enabled is not provided, the user is created as Active. Set Enabled=false to create the user in a Disabled state.

Example request body:

             {
               "FirstName": "Alice",
               "LastName": "Smith",
               "LoginName": "asmith@example.com",
               "CorrespondenceEmailAddress": "asmithnotifications@example.com",
               "GroupIDs": [ 100, 101 ]
             }

On success, returns 201 Created with the full user object in the response body. The Groups field in the response is populated with the groups assigned during creation, including GroupID, GroupName, and IsAdmin for each group. The Location response header contains the URI of the newly created user (e.g., /v1/users/12345).