fix: Remove the same account validation for whatsapp channels (#12811)

## Description

Modified the phone number validation in Whatsapp::ChannelCreationService
to check for duplicate phone numbers across ALL accounts, not just
within the current account.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- Added test coverage for cross-account phone number validation
- Using actual UI flow 
<img width="1493" height="532" alt="image"
src="https://github.com/user-attachments/assets/67d2bb99-2eb9-4115-8d56-449e4785e0d8"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

config/locales/en.yml

@@ -90,6 +90,7 @@ en:
       token_exchange_failed: 'Failed to exchange code for access token. Please try again.'
       invalid_token_permissions: 'The access token does not have the required permissions for WhatsApp.'
       phone_info_fetch_failed: 'Failed to fetch phone number information. Please try again.'
+      phone_number_already_exists: 'Channel already exists for this phone number: %{phone_number}, please contact support if the error persists'
       reauthorization:
         generic: 'Failed to reauthorize WhatsApp. Please try again.'
         not_supported: 'Reauthorization is not supported for this type of WhatsApp channel.'