fix(account-deletion): normalize deleted email suffix and handle collisions safely (#13472)

## Summary
This PR fixes account deletion failures by changing how orphaned user
emails are rewritten during `AccountDeletionService`.

Ref:
https://chatwoot-p3.sentry.io/issues/6715254765/events/e228a5d045ad47348d6c32448bc33b7a/

## Changes (develop -> this branch)
- Updated soft-delete email rewrite from:
  - `#{original_email}-deleted.com`
- To deterministic value:
  - `#{user.id}@chatwoot-deleted.invalid`
- Added reserved non-deliverable domain constant:
  - `@chatwoot-deleted.invalid`
- Replaced the "other accounts" check from `count.zero?` to `exists?`
(same behavior, cheaper query).
- Updated service spec expectation to match deterministic email value
and assert it differs from original email.

## Files changed
- `app/services/account_deletion_service.rb`
- `spec/services/account_deletion_service_spec.rb`

## How to verify
- Run: `bundle exec rspec
spec/services/account_deletion_service_spec.rb`
- Run: `bundle exec rubocop app/services/account_deletion_service.rb
spec/services/account_deletion_service_spec.rb`

app/services/account_deletion_service.rb

@@ -1,4 +1,6 @@
 class AccountDeletionService
+  SOFT_DELETE_EMAIL_DOMAIN = '@chatwoot-deleted.invalid'.freeze
+
   attr_reader :account, :soft_deleted_users
 
   def initialize(account:)
@@ -25,15 +27,11 @@ class AccountDeletionService
 
   def soft_delete_orphaned_users
     account.users.each do |user|
-      # Find all account_users for this user excluding the current account
-      other_accounts = user.account_users.where.not(account_id: account.id).count
+      # Skip users who are still associated with another account.
+      next if user.account_users.where.not(account_id: account.id).exists?
 
-      # If user has no other accounts, soft delete them
-      next unless other_accounts.zero?
-
-      # Soft delete user by appending -deleted.com to email
       original_email = user.email
-      user.email = "#{original_email}-deleted.com"
+      user.email = soft_deleted_email_for(user)
       user.skip_reconfirmation!
       user.save!
 
@@ -47,4 +45,8 @@ class AccountDeletionService
       Rails.logger.info("Soft deleted user #{user.id} with email #{original_email}")
     end
   end
+
+  def soft_deleted_email_for(user)
+    "#{user.id}#{SOFT_DELETE_EMAIL_DOMAIN}"
+  end
 end

spec/services/account_deletion_service_spec.rb

@@ -46,7 +46,8 @@ RSpec.describe AccountDeletionService do
 
         # Reload the user to get the updated email
         user_with_one_account.reload
-        expect(user_with_one_account.email).to eq("#{original_email}-deleted.com")
+        expect(user_with_one_account.email).to eq("#{user_with_one_account.id}@chatwoot-deleted.invalid")
+        expect(user_with_one_account.email).not_to eq(original_email)
       end
 
       it 'does not modify emails for users belonging to multiple accounts' do