Validation rules to prevent switching user companies if items are assigned to them

[snipe]

This is a hopefully better option for #15414, where we check to see that the user does not have items assigned before moving them to another company if FMCS is enabled.

Corrects a defect created in #15284

[what-the-diff]

PR Summary

• Validation Rule Addition to SaveUserRequest.php
  We've included a new check named App\Rules\UserCannotSwitchCompaniesIfItemsAssigned in the SaveUserRequest.php file. This is important to verify a certain condition before a user changes companies.

• New Validation Rule in UserCannotSwitchCompaniesIfItemsAssigned.php
  We've created a new rule in a file UserCannotSwitchCompaniesIfItemsAssigned.php, located in the app/Rules directory. This rule named UserCannotSwitchCompaniesIfItemsAssigned is designed to ensure a user cannot switch companies if they have any items currently assigned to them. This, in turn, secures that these items won't be lost or incorrectly re-assigned during the switching process.