Add validation for brand

[SecondeJK]

This PR adds validation for the mandatory brand parameter in Verify

Motivation and Context

This stops the API request from going out before being validated by Vonage servers.

How Has This Been Tested?

Test amended

Example Output or Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.97%. Comparing base (ba6b02a) to head (b42b1dc).
Report is 38 commits behind head on main.

Files	Patch %	Lines
src/Verify2/Request/EmailRequest.php	50.00%	1 Missing ⚠️
src/Verify2/Request/SMSRequest.php	50.00%	1 Missing ⚠️
src/Verify2/Request/SilentAuthRequest.php	50.00%	1 Missing ⚠️
src/Verify2/Request/WhatsAppInteractiveRequest.php	50.00%	1 Missing ⚠️
src/Verify2/Request/WhatsAppRequest.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #490      +/-   ##
============================================
+ Coverage     79.18%   79.97%   +0.79%     
- Complexity     2371     2643     +272     
============================================
  Files           218      240      +22     
  Lines          6356     7092     +736     
============================================
+ Hits           5033     5672     +639     
- Misses         1323     1420      +97     

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